Pre-heat oven to 350F / 180C

Grease a metal or glass baking pan (8" or 9" should work) generously with butter or olive oil

Add the remaining butter in a heated pan

Add chopped mushrooms, salt, some pepper, and cook on medium-high heat for 3 minutes or until the mushrooms begin to soften

Now add the spinach and cook for another minute, just until it begins to wilt

In a bowl, whisk the eggs along with the milk, cheese, and some salt and pepper

Mix the mushroom-spinach mixture with the egg mixture

Pour into the greased baking tray

Bake in the pre-heated oven for about 20 minutes or until the edges brown and the centre of the frittata has also cooked through</span>

Let it cool slightly and cut into squares or wedges

Serve warm with ketchup or tomato sauce