Set your oven to 375°F (190°C). Grease a 9-inch pie dish or quiche pan.
Cook the Vegetables:
In a medium skillet, heat olive oil over medium heat.
Add the chopped onion and cook until translucent, about 5 minutes.
Add mushrooms and cook until tender and moisture evaporates, about 7–10 minutes.
Stir in spinach and cook until wilted, about 2–3 minutes. Remove from heat and let cool slightly.
Prepare the Egg Mixture:
In a large mixing bowl, whisk together eggs, heavy cream, garlic powder, nutmeg, salt, and pepper until smooth.
Combine and Bake:
Fold the cooked vegetables and crumbled feta into the egg mixture.
Pour the mixture into the prepared pie dish.
Bake for 35–40 minutes, until the quiche is set and the top is golden brown.
Cool and Serve:
Allow to cool for a few minutes before slicing and serving.
