Beat the eggs in a small bowl until smooth. Add a pinch of salt, pepper, and optional milk or cream. Whisk for 30 seconds until no streaks remain.

Heat butter in an 8-inch nonstick skillet over medium-low heat. Swirl the pan to coat the bottom evenly. The butter should melt without browning.

Pour in the eggs and let them sit for 30 seconds. Use a spatula to gently push the edges toward the center, tilting the pan so uncooked egg flows to the sides. Repeat until the eggs are mostly set but still slightly wet on top, about 3 minutes.

Add spinach and feta to one half of the omelet. Sprinkle the chopped spinach and crumbled feta over half the surface. Let it sit for 30 seconds so the spinach starts to wilt.

Fold the omelet in half and slide onto a plate. Use the spatula to fold the empty half over the filled side. Turn off the heat and let the omelet rest in the pan for 30 seconds to finish cooking gently.