Preheat your oven to 375°F (190°C). Lightly grease a 9-inch pie dish or a 9×9-inch baking dish with cooking spray or butter. Set it aside.
Heat the olive oil or butter in a medium skillet over medium heat. Add the diced onion and sliced mushrooms, and cook for about 4–5 minutes, stirring occasionally, until the mushrooms release their liquid and it mostly evaporates. Add the chopped spinach and cook for another 1–2 minutes, just until wilted. Remove from heat and let the vegetables cool for a couple of minutes.
In a large bowl, crack the eggs and whisk them until the yolks and whites are fully combined. Add the milk, salt, black pepper, garlic powder, and dried herbs if using. Whisk again until smooth and slightly frothy.
Stir about ¾ cup of the shredded cheese into the egg mixture, reserving the remaining ¼ cup for the top. Fold in the sautéed vegetables until evenly distributed.
Pour the mixture into the prepared baking dish. Sprinkle the reserved cheese on top.
Bake until the center is set but still has a slight wobble, about 35–40 minutes. Let it rest for a few minutes before slicing.
The aroma of eggs and melted cheese filling your kitchen is the kind of simple pleasure that makes cooking so rewarding.