Preheat the oven to 190°C (375°F).

In an oven-safe skillet, heat olive oil over medium heat. Add mushrooms and cook until soft.

Add spinach and cook until wilted.

In a bowl, whisk eggs, milk, salt, and pepper. Pour into the skillet.

Sprinkle with cheese if using.

Cook on the stovetop for 2-3 minutes, then transfer to the oven and bake for 10-12 minutes, or until the eggs are set.