01 - Preheat oven to 350°F. Grease a 12-cup muffin tin with cooking spray or line with paper liners.
02 - Heat olive oil in a skillet over medium heat. Add diced onion and cook for 2-3 minutes until softened. Stir in minced garlic and chopped spinach; cook until spinach is wilted, about 2 minutes. Remove from heat.
03 - In a large mixing bowl, whisk together eggs, milk, salt, pepper, and nutmeg until well combined.
04 - Fold the cooked spinach mixture and crumbled feta cheese into the egg mixture. Add shredded cheese if using.
05 - Divide the mixture evenly among the prepared muffin cups, filling each about three-quarters full.
06 - Bake for 18-20 minutes or until the muffins are set and lightly golden on top.
07 - Let cool in the tin for 5 minutes before removing. Serve warm or at room temperature.