Preheat oven to 350. Dice mushrooms and saute with spinach. Fill each muffin cup evenly with egg, cheese and mushroom & spinach saute. Gently stir the contents of each cup. Each cup should be about 3/4 full.
Bake at 350 for about 25-35 minutes. When the muffins are done, loosen them with a knife before taking them out of the muffin pan. Remove from muffin pan and let cool for about 20 minutes on wire rake before transferring into the refrigerator or freezer.