01 - Preheat oven to 350°F. Grease a 12-cup muffin tin or line with paper liners.
02 - Heat olive oil in a skillet over medium heat. Add onion and cook 2-3 minutes until softened. Stir in garlic and spinach; cook until spinach is wilted. Remove from heat.
03 - In a large bowl, whisk eggs with milk, salt, pepper, and nutmeg until well combined.
04 - Fold in the cooked spinach mixture and crumbled feta (and shredded cheese if using).
05 - Divide the mixture evenly among the prepared muffin cups.
06 - Bake for 18-20 minutes until set and lightly golden on top.
07 - Let cool for a few minutes before removing from the tin. Serve warm or at room temperature.