Spray a 12 cup muffin tin with cooking spray and preheat oven to 375 degrees, then heat olive oil over medium-high heat in large skillet until shimmering. Add mushrooms, onion, and sea salt to skillet and saute, stirring occasionally, for five minutes or until mushrooms are softened and have released most of their liquid.
Remove mushroom & onion mixture with a slotted spoon and distribute it evenly among the muffin tin cups, leaving the liquid behind in the pan. Add spinach, pepper, and crushed red pepper to the same skillet, and saute until spinach is completely wilted and bright green. Drain spinach, press out as much liquid as possible, and distribute the cooked spinach evenly among the muffin tin cups.
Beat eight eggs until well combined and pour them evenly into the muffin tin cups on top of the veggies. Break up four slices of cheese into smaller pieces, then top each egg muffin with cheese. Bake at 375 degrees for 25 minutes until your little frittatas are puffed up and browned, then let cool for about five minutes. Loosen edges with a butter knife before removing from pan.
I used Swiss cheese here because I had it on hand, but sharp cheddar would actually be much better in order to add a bit more flavor. It's also easy to change up the veggies in these to your own liking (try red bell pepper or broccoli...). Muffin Tin Frittatas are low carb and naturally gluten free, and can freeze well for later -- try them with a little hot sauce on the side.
Spinach Mushroom Muffin Tin Mini Frittatas from Mashup Mom: https://www.mashupmom.com/spinach-mushroom-muffin-tin-mini-frittatas/