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