Preheat your oven to 350°F (175°C).

In a skillet over medium heat, add the olive oil. Once hot, toss in the onions and cook until translucent, about 5 minutes.

Add the sliced mushrooms and sauté until browned, around 5 more minutes.

Stir in the spinach and cook until wilted (if using fresh), then remove from heat.

In a large bowl, whisk together the eggs and milk until combined. Season with salt and black pepper to taste.

Add the sautéed vegetable mixture and shredded cheese into the egg mixture and stir to combine well.

Pour the mixture into a greased 9-inch pie dish or another oven-safe dish, spreading it evenly.

Place it in the preheated oven and bake for 20-25 minutes, or until the eggs are set and slightly golden on top.

Allow it to sit for a few minutes before slicing and serving warm.