Vegetable Quiche

Preheat the oven to 375°F (190°C). Place a baking sheet on the middle rack to preheat with the oven.
Fit the pie crust in a 9-inch pie dish. Prick the base with a fork and pre-bake for 8–10 minutes until just matte and lightly set. Set aside to cool slightly.
Heat olive oil in a skillet over medium heat. Sauté onion and red bell pepper for 2–3 minutes until softened. Add mushrooms with a pinch of salt and cook until they release and reabsorb their moisture, 3–4 minutes.
Stir in spinach and cook just until wilted, about 1 minute. Remove the pan from heat and let vegetables cool for a couple of minutes.
In a mixing bowl, whisk together eggs, milk (or half-and-half), salt, black pepper, and nutmeg (if using) until smooth but not foamy.
Sprinkle half of the shredded cheese over the par-baked crust. Spread the sautéed vegetables evenly on top.
Pour the egg mixture over the vegetables. Sprinkle the remaining cheese on top. Add optional cherry tomatoes or zucchini slices and herbs if using.
Set the pie dish on the preheated baking sheet and bake for 35–40 minutes, or until the edges are puffed, the top is lightly golden, and the center is just set with a slight wobble.
Cool the quiche for 10 minutes before slicing. Serve warm, at room temperature, or chilled.