Preheat your oven to 375°F. While it heats, prepare all your ingredients: slice the mushrooms into 1/4-inch pieces, dice the sweet onion into 1/4-inch pieces, mince the garlic, strip the thyme leaves from their stems, and shred the Gruyère cheese if not already done. This prep work ensures you won't be rushed once cooking begins and allows you to focus on building flavors.

Heat the olive oil in a large skillet over medium-high heat. Add the sliced mushrooms and cook for 8 minutes, stirring occasionally, until they release their moisture and begin to brown—this develops deep umami flavors. Add the diced onion and minced garlic, cooking for another 5 minutes until the onion is soft and translucent. Finally, add the fresh spinach and cook for 1-2 minutes just until wilted. I like to use fresh garlic here because it infuses the vegetables with much more flavor than pre-minced garlic from a jar. Remove the skillet from heat and set aside to cool slightly.

In a medium bowl, whisk together the room-temperature eggs, milk, half-and-half, Dijon mustard, fresh thyme leaves, salt, and pepper until well combined and smooth. The mustard adds a subtle depth and helps bind the filling together. Room-temperature eggs integrate more smoothly into the custard, creating a more uniform texture throughout the quiche.

Pour the custard mixture from Step 3 into the skillet with the cooled sautéed vegetables from Step 2. Gently fold in the shredded Gruyère cheese, stirring just until combined. Be careful not to overmix—gentle folding keeps the filling tender and prevents the cheese from becoming stringy. This combined filling is now ready to transfer to the quiche pan.

Lightly oil a 9-inch pie pan or quiche dish with a thin coating of oil, making sure to coat the bottom and sides evenly to prevent sticking. Pour the combined filling from Step 4 into the prepared pan, spreading it evenly. The filling should sit just below the rim to allow for slight puffing during baking.

Place the quiche in the preheated 375°F oven and bake for 30 minutes, until the filling is set but still slightly jiggly in the very center (about a 2-inch circle in the middle), and the top is lightly golden. The residual heat will continue cooking the center as it cools. If the top begins to brown too quickly, tent loosely with foil for the final 10 minutes of baking.

Remove the quiche from the oven and let it rest at room temperature for 10 minutes before slicing. This resting period allows the custard to set fully and makes slicing much cleaner and easier. Serve warm or at room temperature—the quiche is delicious either way!