Preheat the oven to 350°F (175°C) and lightly grease a 12-cup muffin tin with butter or olive oil. A silicone muffin pan works especially well for easy removal.
Crack the eggs into a large mixing bowl.
Add the sea salt and black pepper, then whisk until smooth and fully combined.
Stir in the chopped spinach until evenly distributed.
Divide the egg mixture evenly among the prepared muffin cups, filling each about halfway.
Sprinkle the crumbled feta cheese evenly over each muffin cup.
Bake the Spinach Egg Muffins for 18–20 minutes, or until the centers are fully set and the tops are lightly golden.
Remove from the oven and allow the muffins to cool for a few minutes before carefully removing them from the pan.
Serve warm or allow to cool completely before storing.