01 - Whisk together the eggs, milk, salt, black pepper, and nutmeg in a bowl until well combined and uniform in color.
02 - Heat butter or olive oil in a nonstick skillet over medium heat. Add the shallot or onion and sauté until translucent, approximately 1-2 minutes.
03 - Add the chopped spinach to the skillet and cook, stirring constantly, until wilted and reduced in volume, about 1 minute.
04 - Pour the egg mixture into the skillet, tilting the pan to distribute evenly over the spinach and shallot mixture.
05 - Cook undisturbed for 2-3 minutes until edges begin to set. Use a spatula to gently lift edges, allowing uncooked egg to flow underneath.
06 - Sprinkle crumbled feta cheese evenly over one half of the omelet.
07 - When eggs are mostly set but the top remains slightly runny, fold the omelet in half. Cook for an additional 30 seconds to melt cheese, then slide onto a serving plate.
08 - Serve immediately, optionally garnished with additional crumbled feta or fresh herbs such as dill or parsley.