Preheat the oven to 350 degrees and spray a muffin tin with nonstick spray.
In a large mixing bowl, whisk the eggs with the milk. Sprinkle with salt and pepper as desired. (The salt and pepper can also be added at the end.)
Evenly spread the egg mixture between 12 muffin tins. In each one, add an equal amount of the chopped spinach, turkey sausage, and mushrooms. Add 1 tsp of shredded cheese on the top of each mini quiche.
Bake the eggs in the oven for 20-25 minutes or until the eggs are cooked all the way through. The tops will be slightly browned. Let cool and refrigerate for later!