01 - Preheat your oven to 350°F. Grease a 12-cup muffin tin thoroughly 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 and translucent. Stir in minced garlic and chopped spinach; cook until spinach is completely wilted, about 2 minutes. Remove from heat and set aside to cool slightly.
03 - In a large mixing bowl, whisk together eggs, milk, salt, black pepper, and nutmeg until fully combined and slightly frothy.
04 - Fold the cooked spinach mixture and crumbled feta into the egg base. Add shredded cheese if using, stirring gently to distribute evenly throughout.
05 - Divide the mixture evenly among the 12 prepared muffin cups, filling each about three-quarters full.
06 - Bake for 18-20 minutes until the muffins are set in the center and lightly golden on top. A toothpick inserted into the center should come out clean.
07 - Let the muffins cool in the tin for 5 minutes before removing. Serve warm or at room temperature.