Whisk together eggs and milk until they are well combined. Season to taste with salt and pepper.
Melt butter in a large saute pan, and saute mushrooms until lightly browned. Place spinach in pan and saute until slightly wilted.
Spray a non-stick mini muffin pan well with cooking spray. Divide mushrooms and spinach equally between all 24 cups. Place approximately 1 teaspoon of shredded cheese on each muffin cup. Pour egg mixture in all muffin cups.
Bake at 325 for 15 - 20 minutes, or until eggs are set and cooked to your liking.