Heat olive oil in a pan over medium heat. Add chopped onion and sauté until softened, about 3-4 minutes.
Add chopped spinach to the pan and cook until wilted, about 2-3 minutes. Set the mixture aside to cool slightly.
Preheat the oven to 375°F (190°C). Grease a muffin tin and set aside.
Whisk eggs in a bowl, then add salt, pepper, and the spinach-onion mixture. Stir in crumbled feta cheese.
Pour the egg mixture into the muffin tin, filling each cup about ¾ full. Bake for 15-18 minutes, or until eggs are set and lightly golden.
Allow the muffins to cool for a few minutes before serving. Enjoy warm or store for later.