Saute mushrooms in butter in large saute pan on medium heat for several minutes until they give off water. Add spinach and tomatoes and saute for 2-3 minutes more.
Reduce heat to medium low.
Meanwhile, whisk eggs and milk together and add salt and pepper to taste.
Pour egg mixture over vegetables and evenly distribute vegetables at bottom of pan. Let sit for 2-3 minutes.
Place in oven and cook for 8-10 minutes until set.
Recipe by Weekend Table at https://weekendtable.com/2013/07/23/mushroom-tomato-and-mozzarella-frittata/