01 - Whisk together eggs, milk, salt, pepper, and nutmeg in a bowl until fully combined and frothy.
02 - Heat butter or olive oil in a nonstick skillet over medium heat. Add shallot or onion and cook until translucent, approximately 1-2 minutes.
03 - Add chopped spinach to the skillet and stir constantly until wilted, about 1 minute.
04 - Pour egg mixture into the skillet, tilting pan to distribute evenly over the spinach and aromatics.
05 - Cook undisturbed for 2-3 minutes until edges begin to set. Gently lift edges with spatula to allow uncooked egg to flow underneath.
06 - Sprinkle crumbled feta cheese evenly over one half of the omelet.
07 - Fold omelet in half when eggs are mostly set but top remains slightly runny. Cook 30 seconds longer, then slide onto plate.
08 - Serve immediately, garnished with additional feta or fresh herbs if desired.