01 - Set oven temperature to 375°F.
02 - Roll out the pastry and place it into a 9-inch tart pan. Trim excess dough and prick the base with a fork. Chill in refrigerator for 10 minutes.
03 - Line pastry with parchment paper and fill with baking weights or dried beans. Bake for 12 minutes. Remove weights and paper, then bake for 5 more minutes until lightly golden. Cool slightly.
04 - Heat olive oil in skillet over medium heat. Sauté onion, zucchini, and bell pepper for 5 minutes. Add mushrooms and cook for 3 minutes. Stir in spinach and cook until wilted. Remove from heat and cool slightly.
05 - In a medium bowl, whisk eggs, whole milk, heavy cream, salt, black pepper, and nutmeg until smooth.
06 - Spread sautéed vegetables evenly over baked crust. Sprinkle Gruyère and optional feta cheese on top.
07 - Pour the egg mixture evenly over the vegetable and cheese filling.
08 - Bake for 35 to 40 minutes until custard is set and top is golden brown.
09 - Let the quiche cool for 10 minutes before slicing. Serve warm or at room temperature.