Preheat your oven to 375°F (190°C). Lightly grease a 9-inch pie dish with butter or cooking spray to prevent sticking.

Heat olive oil in a large skillet over medium heat. Add chopped yellow onion and diced red bell pepper. Sauté for 5-7 minutes until softened. Add diced zucchini and sliced mushrooms, cooking for another 3-5 minutes until tender-crisp. Stir in minced garlic and chopped baby spinach. Cook until the spinach wilts, about 1-2 minutes. Remove the skillet from heat and let the vegetables cool slightly.

In a large mixing bowl, whisk together the large eggs, whole milk, salt, black pepper, dried thyme, and a pinch of nutmeg until all ingredients are well combined and slightly frothy.

Sprinkle approximately half of the shredded Gruyere cheese evenly over the bottom of the prepared pie dish. Spoon the cooled sautéed vegetable mixture over the layer of cheese.

Carefully pour the whisked egg mixture evenly over the vegetables and cheese in the pie dish, ensuring an even distribution.

Sprinkle the remaining half of the Gruyere cheese and all of the grated Parmesan cheese over the top of the liquid quiche mixture. This will create a nice golden crust.

Carefully place the pie dish in the preheated oven. Bake for 40-45 minutes, or until the quiche is set in the center (a knife inserted near the center should come out clean) and the top is beautifully golden brown.

Remove the quiche from the oven and let it rest on a wire rack for 5-10 minutes before slicing and serving. This resting period allows the quiche to fully set and makes for cleaner slices.