Preheat the oven to 350°F and grease a 12-cup muffin tin. (If you have a silicone muffin tin I would suggest using it as it is easier to remove the muffins.)
In a large bowl, crack eggs, whisk together and season with salt and pepper. Add chopped spinach and whisk again to combine.
Divide egg mixture evenly across the muffin tins, filling them halfway, and sprinkle with crumbled feta cheese. Depending on the size of your eggs you may have a little extra of the mixture, so you can make a few more.
Transfer the muffin tin to the oven and bake for 18-20 minutes until the centre of each egg muffin is set.
Once cooked, serve immediately or store in an airtight container in the fridge for up to 5 days.