An easy delicious low-cal, low-carb, breakfast. You can make a batch in advance and freeze them. Just microwave for a healthy breakfast whenever you're in a hurry.
Heat a skillet over medium high heat. When it's hot, spray with nonstick cooking spray and add the mushrooms. Cook for about 5 minutes, stirring occasionally, until mushrooms give up their liquid and are tender and starting to brown. Transfer mushrooms to a small bowl.
Heat 1 ½ teaspoons of the olive oil in the skillet and add the sliced onions. Cook for 5 minutes or until slightly softened and translucent. Add the salt, pepper and red pepper flakes and stir. Add the spinach to the onions and simmer with the lid on, stirring occasionally, until spinach wilts and onions are soft. Add the mushrooms back to the pan and stir to combine.
Crack the eggs into a large bowl and whisk until frothy.
Spray a standard muffin tin liberally with vegetable spray and set aside.
Add the mozzarella to the vegetables and stir to combine. Ladle half of the eggs into the muffin tins, about 2-3 tablespoons per tin. Divide the vegetable and cheese mixture between each of the muffin tins. Ladle the remaining eggs over the vegetable mixture. Top each muffin tin with a sprinkle of mozzarella and a teaspoon of parmesan.
Bake for 20-25 minutes, or until eggs are set. Turn the oven to broil and cook for an additional 2-4 minutes to brown the tops of the egg cups. Remove from oven and use a sharp knife to rim each muffin tin and loosen the egg cup. Transfer to a serving platter, or if you're freezing for future use, wrap each egg cup individually in plastic wrap and store in a gallon sized freezer bag.
To reheat in the microwave, fold a paper towel on a microwaveable plate and rest the frittata on top. Microwave for 1 minute on 50% power. Then microwave in 20 second bursts at full power, until heated through. (usually 2-3 bursts). In case you're wondering about the paper towel, it soaks up excess liquid from the mushrooms and spinach.
Notes
Leftovers keep for 3-5 days when refrigerated or you can freeze them for up to two months.