01 - Whisk together the 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 the shallot or onion and sauté until translucent, about 1–2 minutes.
03 - Add the spinach and cook, stirring, until wilted, about 1 minute.
04 - Pour the egg mixture into the skillet, tilting to distribute evenly over the spinach and shallot/onion.
05 - Cook undisturbed for 2–3 minutes until the edges begin to set. Use a spatula to gently lift the edges, letting any uncooked egg flow underneath.
06 - Sprinkle crumbled feta evenly over one half of the omelet.
07 - When the eggs are mostly set but the top is slightly runny, fold the omelet in half. Cook for another 30 seconds, then slide onto a plate.
08 - Serve immediately, garnished with extra feta or fresh herbs if desired.