Preheat Eggs Plus Pan according to directions. Add a small amount of olive oil or butter to each well on the pan, then add mushrooms to each cavity. Saute 1 minute, then add egg mixture (divided evenly between wells). Immediately add spinach to each cavity, using a spatula to gently push the spinach into the liquid egg. Cook 2 minutes. Add 1 Tbsp of water to the patented Reservoir in the center of the pan, then sprinkle feta cheese on top of each frittata. Place lid onto the pan. Water will evaporate inside, and the steam will help complete the cooking of the egg and also melt the cheese. Cook to desired doneness, 1-2 minutes more. Immediately slide frittatas onto individual plates, sprinkle with salt and pepper to taste, and serve with fresh fruit and warm toast.