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