Preheat oven to 350°F (175°C).
Lightly grease a 9x9-inch baking dish.
Heat olive oil in a skillet over medium heat.
Sauté mushrooms and onion until softened.
Add chopped spinach and cook until wilted.
In a bowl, whisk eggs and milk together.
Stir in garlic powder, salt, and black pepper.
Spread vegetable mixture evenly in baking dish.
Pour egg mixture over vegetables.
Sprinkle mozzarella and cheddar cheese on top.
Bake for 30–35 minutes until eggs are set and top is golden.
Let cool slightly before slicing and serving.
