Prepare your ingredients by cleaning and slicing the mushrooms and dicing the onion.

Heat olive oil in a large skillet over medium heat. Sauté the onions until translucent; add sliced mushrooms and cook until golden brown.

Stir in fresh spinach until wilted, then sprinkle crumbled feta on top.

In a separate bowl, whisk together eggs with salt and pepper until frothy.

Pour the egg mixture over the sautéed vegetables in the skillet. Cook undisturbed for 5-7 minutes until edges set but center remains slightly runny.

Transfer skillet to a preheated oven at 375°F (190°C) for 10-15 minutes until fully cooked and lightly browned.