Preheat the oven to 400 degrees F.
Grease your muffin pan and set aside.
Wash the spinach, and twist off any long steams, you can also use chopped spinach, but I like to leave mine whole. Pack each muffin cup with spinach, then sprinkle 1-2 tbsp of crumbled feta cheese.
Then add the eggs. I like to crack one egg into each cup, leaving the egg whole.
If you are using egg whites, whisk them for a minute to two in a large bowl, then pour them over the spinach and feta, doing your best to put the same amount of egg in each cup. If you are using scrambled eggs, in a mixing bowl whisk together your eggs with a 1/4 cup of milk, then pour the eggs over the spinach and feta, again doing your best to put an equal amount in each cup.
Season with salt, pepper, and crushed red pepper. Place the finished egg cups in the preheated oven and back for 10-15 minutes or until cooked through.
If you are cooking whole eggs like I did, you can pull them out a little earlier, if you like soft yolks. However, I would recommend cooking them all the way through if you plan on keeping them in the fridge to eat throughout the week.
Allow the egg muffines to cool slightly, before removing them from the muffin tin.
Serve with a buttered piece of toast or an English muffin. Enjoy!