Frittata with spinach and mushrooms

Ingredients
6 eggs
60 ml milk
100 g grated cheddar cheese
1 onion, thinly sliced
115 g white mushrooms, sliced
40 g butter
50 g spinach

Preparation
1-Place the rack in the middle of the oven.
Preheat the oven to 180 °C (350 °F). Butter a 20-cm (8-inch) square baking pan.
2-In a large bowl, whisk together the eggs and milk. Add the cheese. Season with salt and pepper.
3-In a large non-stick skillet over medium heat, brown the onion and mushrooms in the butter. Season with salt and pepper. Add the spinach and cook for 1 minute, stirring constantly. Transfer to the egg mixture. Mix and pour into the baking dish.
4-Bake for 25 minutes or until the frittata is golden brown and lightly blown. Cut the frittata into four squares and remove from the pan with a spatula. Serve hot or cold.