Spinach and Feta Mini Frittatas

Preheat oven to 375F and spray a mini muffin pan with nonstick spray. Set aside.Place spinach in a steamer set over boiling water, cover, and steam until just wilted, about 1 minute.Melt butter in a small pan set over medium heat. Add the mushrooms and cook until soft, about 5 to 6 minutes. Transfer to a separate bowl and add the wilted spinach and chopped green onions, mixing well.In a medium-sized mixing bowl, whisk together eggs, milk, feta cheese, salt, and pepper. Divide the spinach, onion, and mushroom mixture evenly among the muffin tins, and use a ladle to spoon the egg mixture over top, filling about ¾ of the way full.Bake for 15-20 minutes until eggs have set and edges begin to turn golden brown. Remove from oven and allow to cool for 5 minutes before removing from tins and serving.