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