Preheat your oven to 350°F. While it heats, prepare all ingredients: mince the garlic, finely dice the onion into 1/4-inch pieces, thinly slice the mushrooms for even cooking, roughly chop the fresh spinach, and grate the Parmesan cheese fresh if possible—this makes a noticeable difference in flavor and melting quality. Having everything prepped and ready will make the cooking process smooth and stress-free.

In a large skillet over medium-high heat, sauté the diced onion and minced garlic until fragrant, about 1 minute. Add the sliced mushrooms and cook for 5-7 minutes, stirring occasionally, until they release their moisture and begin to brown slightly. The mushrooms will shrink significantly as they cook, which is exactly what you want—this concentrates their earthy flavor. Once the mushrooms are tender and any excess liquid has mostly evaporated, add the chopped spinach and stir constantly for 2-3 minutes until it wilts completely and any remaining liquid cooks off.

In a large mixing bowl, combine the 4 large eggs, 4 egg whites (room temperature eggs bind better than cold ones), 1 cup milk, salt, and minced garlic. Whisk vigorously for about 20 seconds until the mixture is well combined and slightly frothy—this incorporates air and ensures an evenly cooked, creamy quiche. Don't overmix beyond this point, as excessive beating can make the texture tough.

Spread the sautéed vegetable mixture (from Step 2) evenly across the bottom of a 9-inch pie pan or similar-sized baking dish. I recommend spreading it in a single, even layer to ensure every bite has the same proportion of vegetables. Sprinkle the shredded cheddar cheese and 1/4 cup of the Parmesan cheese over the vegetables. Pour the egg custard mixture (from Step 3) slowly over the top, allowing it to settle and distribute evenly around the vegetables. The egg should come up to about three-quarters full in the pan.

Place the assembled quiche in the preheated 350°F oven and bake for 35 minutes, until the edges are set but the center still has a slight jiggle when you gently shake the pan—this means the center will be creamy rather than rubbery. The top should be lightly golden brown. If the top begins to brown too quickly before the center is set, loosely tent it with aluminum foil for the remaining baking time.

Remove the quiche from the oven and immediately sprinkle the remaining 1/4 cup of Parmesan cheese over the top while it's still hot, so it melts slightly and adheres to the surface. Allow the quiche to rest at room temperature for 15 minutes before slicing and serving—this resting period allows the egg to fully set and makes it much easier to cut clean slices without the custard running.