01 - Preheat your oven to 350°F. Grease a 12-cup muffin tin thoroughly or line with paper liners for easy removal.
02 - Heat olive oil in a skillet over medium heat. Add diced onion and sauté for 2-3 minutes until softened and translucent. Stir in minced garlic and chopped spinach; cook until spinach is completely wilted, about 2-3 minutes. Remove from heat and set aside to cool slightly.
03 - In a large mixing bowl, crack the eggs and whisk until well blended. Add milk, salt, black pepper, and nutmeg (if using). Whisk thoroughly to combine all seasonings.
04 - Fold the cooled spinach mixture into the egg base. Add crumbled feta cheese and shredded cheese if desired. Mix gently until evenly distributed throughout the mixture.
05 - Divide the egg and vegetable mixture evenly among the 12 prepared muffin cups, filling each about three-quarters full. Use a spoon or small measuring cup for consistent portions.
06 - Place the muffin tin in the preheated oven and bake for 18-20 minutes. The muffins are done when they are set in the center and lightly golden on top. A knife inserted in the center should come out clean.
07 - Allow the muffins to cool in the tin for 5 minutes before removing. Run a knife around the edges if needed to loosen. Serve warm or at room temperature.