Preheat the oven to 350°F (175°C).
Grease a 12-cup muffin tin with butter or olive oil.
In a large mixing bowl, crack the eggs and whisk until smooth.
Add sea salt and black pepper and whisk again.
Stir in the chopped spinach until evenly distributed.
Divide the egg mixture evenly among the muffin cups, filling each about halfway.
Sprinkle crumbled feta cheese evenly over each muffin cup.
Transfer the muffin tin to the oven and bake for 18–20 minutes, or until the centers are fully set.
Remove from the oven and allow the Spinach Egg Muffins to cool for a few minutes before serving.
Enjoy immediately or store for later meal prep.