Best Cheesy Zucchini Leek Casserole

I’ve always loved finding new ways to use up the extra vegetables from my garden. When zucchini season hits and I’m swimming in more squash than I know what to do with, this casserole becomes my go-to dinner solution. It’s the kind of meal that makes good use of simple ingredients without creating a mess of pots and pans to clean up afterward.

What I really like about this zucchini and leek combination is how the flavors work together without trying too hard. The leeks add just enough interest to make the zucchini feel like something special, and the whole dish comes together in one pan. I often prep everything while I’m already in the kitchen making lunch, then pop it in the oven when dinnertime rolls around.

Not sure what to do with those extra zucchini sitting on your counter? Or maybe you’re just looking for a simple vegetable dish that’ll actually get eaten? This casserole might be exactly what you need.

What Kind of Zucchini Should I Use?

For a casserole, medium-sized zucchini (about 7-8 inches long) are your best bet since they’re tender but still hold their shape when cooked. The really large, oversized zucchini you might find in late summer tend to be more watery and have larger seeds, so it’s best to skip those for this dish. When shopping, look for zucchini that feel firm and heavy for their size, with smooth, shiny dark green skin. If you’re prepping ahead, you can slice your zucchini and lay the pieces on paper towels for about 15 minutes to remove excess moisture – this will help keep your casserole from becoming too watery.

Options for Substitutions

This cozy casserole is pretty adaptable – here’s what you can swap if needed:

• Zucchini: You can easily swap zucchini with yellow summer squash, or even eggplant (just salt and drain it first to remove excess moisture). Mexican squash works great too!
• Leeks: No leeks? Use a combination of green onions and regular onions instead. You’ll need about 1 cup of chopped green onions plus 1/2 cup of regular onions to replace the leeks.
• Bacon: Bacon can be replaced with ham, pancetta, or for a vegetarian version, try smoked paprika or liquid smoke for that smoky flavor. You could also use turkey bacon.
• Mozzarella and Cheddar: Feel free to mix up the cheese combo – provolone, Monterey Jack, or Gruyere would all work well. Just make sure to keep a mix of a melty cheese (like mozzarella) and a flavorful cheese (like cheddar) for the best results.
• Tomatoes: Fresh tomatoes can be swapped with canned diced tomatoes (just drain them first), or even roasted red peppers for a different but tasty twist.

Watch Out for These Mistakes While Cooking

The biggest challenge when making zucchini casserole is dealing with excess moisture – zucchini releases a lot of water during cooking, which can make your casserole soupy instead of creamy. To prevent this, salt your sliced zucchini and let it sit for 20-30 minutes, then pat it dry with paper towels before assembling the casserole. Another common mistake is undercooking the leeks, which can leave them tough and stringy – make sure to sauté them until they’re completely soft and slightly golden, about 10-15 minutes. When layering your ingredients, avoid adding too much cheese in one spot, as this can create greasy pools – instead, spread it evenly throughout the layers for better melting and distribution. For the crispiest bacon topping, cook it separately until almost done, then add it to the top of your casserole during the final 10 minutes of baking.

What to Serve With Zucchini Leek Casserole?

This cheesy veggie casserole makes a fantastic main dish that pairs really well with simple sides to round out your meal. A crusty piece of French bread or warm garlic bread is perfect for soaking up all those tasty juices at the bottom of the dish. Since the casserole is pretty rich with cheese and bacon, I like to balance it with a light mixed green salad dressed with a simple vinaigrette. For a heartier meal, you could serve it alongside some roasted chicken or grilled fish – the flavors work great together and make the veggie-based casserole feel even more filling.

Storage Instructions

Keep Fresh: This zucchini leek casserole stays good in the fridge for up to 4 days when stored in an airtight container. The flavors actually get better after a day as everything melds together nicely. Just keep in mind that the zucchini might release a bit more liquid as it sits.

Freeze: You can freeze portions of this casserole in freezer-safe containers for up to 3 months. While the texture of the zucchini might be a bit softer after freezing, it still makes for a convenient meal prep option. Just make sure to cool it completely before freezing.

Reheat: To warm up your casserole, pop it in the oven at 350°F for about 20-25 minutes, or until heated through. If using the microwave, heat in 1-minute intervals, stirring between each. To help prevent the cheese from getting too rubbery, try covering it while reheating.

Preparation Time	15-20 minutes
Cooking Time	20-25 minutes
Total Time	35-45 minutes
Level of Difficulty	Easy

Estimated Nutrition

Estimated nutrition for the whole recipe (without optional ingredients):

• Calories: 800-900
• Protein: 40-50 g
• Fat: 60-70 g
• Carbohydrates: 30-40 g

Ingredients

• Fresh zucchini
• Leek stalks
• Cooked bacon pieces
• Mozzarella, shredded
• Cheddar, grated
• Tomatoes
• Caramelized onion

Step 1: Preheat the Oven and Prepare the Ingredients

• fresh zucchini
• leek stalks
• tomatoes
• caramelized onion

Preheat your oven to 375°F (190°C) while you prepare the vegetables.

Slice the zucchini and leek stalks into even rounds, and finely chop the onion.

If using whole tomatoes, chop them as well.

Having everything prepped in advance will make the cooking process smoother.

Step 2: Cook the Bacon

• cooked bacon pieces

In a large skillet over medium heat, cook the bacon pieces until they are crispy.

Once done, remove the bacon with a slotted spoon and set it aside on a plate lined with paper towels.

Make sure to leave the bacon fat in the skillet, as it will add great flavor to the vegetables.

Step 3: Caramelize the Onions and Leeks

• leek stalks (sliced from Step 1)
• caramelized onion (chopped from Step 1)
• tomatoes (chopped from Step 1)
• bacon fat (from Step 2)

Add the sliced leeks and chopped onions to the skillet with the reserved bacon fat.

Cook over medium heat, stirring occasionally, until the vegetables become soft and caramelized.

This should take about 10-12 minutes.

If using raw tomatoes, add them halfway through to allow them to soften.

I like letting the onions get really golden for an extra boost of sweetness.

Step 4: Cook the Zucchini and Combine with Bacon

• fresh zucchini (sliced from Step 1)
• cooked bacon pieces (crumbled from Step 2)
• vegetable mixture (from Step 3)

Add the sliced zucchini to the skillet with the caramelized vegetables and cook for 3-5 minutes, just until the zucchini starts to soften.

Season the mixture with salt and pepper to taste.

Crumble the crispy bacon (from Step 2) and mix it into the skillet with the vegetables.

Stir everything together until well combined.

Step 5: Assemble and Bake the Gratin

• mozzarella, shredded
• cheddar, grated
• vegetable and bacon mixture (from Step 4)

Transfer the vegetable and bacon mixture from the skillet into a cast-iron skillet or shallow baking dish, spreading it evenly.

Sprinkle a generous layer of shredded mozzarella and grated cheddar cheese over the top.

Place the dish in the preheated oven and bake for 20-25 minutes, or until the cheese is melted, bubbly, and golden brown.

For extra color, I sometimes broil the top for the last 2 minutes.

Step 6: Cool and Serve

Remove the gratin from the oven and let it cool slightly for a few minutes before serving.

This helps the flavors meld and the cheese set.

Serve the gratin warm—it’s delicious on its own, or you can pair it with a fresh green salad or some low-carb bread.