Sauté the Spinach: Heat a small nonstick skillet (about 8 inches wide) over medium-high heat. Drizzle in the olive oil, and once it begins to shimmer, add the baby spinach. Season lightly with salt and pepper. Toss gently and cook for about 1 minute, just until the spinach wilts. Transfer the cooked spinach to a small plate and set it aside.
Beat the Eggs: Crack the eggs into a mixing bowl. Season with a pinch of salt and pepper, then whisk briskly using a fork or mini whisk until the mixture is smooth and fully blended.
Cook the Egg Base: Return the pan to medium heat and melt the butter, swirling it to coat the bottom evenly. Pour the beaten eggs into the pan and immediately swirl or gently shake the pan to spread them into a uniform layer. Let the eggs cook undisturbed for about 30 seconds until the bottom sets slightly.
Form the Omelet: Use a flexible rubber spatula to lift the outer edges of the omelet, allowing the uncooked eggs to flow underneath. Continue cooking for another 30 seconds or until most of the eggs are set, but the top remains slightly glossy.
Add the Filling and Fold: Sprinkle the wilted spinach and crumbled feta over one half of the omelet. Let it warm for 20–30 seconds so the cheese begins to soften and melt. Carefully fold the other half of the omelet over the filling to create a semi-circle.
Plate and Garnish: Slide the omelet onto a serving plate. If desired, sprinkle with fresh chopped chives for a pop of flavor and color. Serve hot and enjoy immediately.