2teaspoonsminced fresh chives to garnish (optional)
Heat an 8-inch omelet pan, or shallow skillet, preferably nonstick, over medium-high heat. Add the olive oil, then when hot add the spinach, season lightly with salt and pepper, and toss for one minute until the spinach is wilted. Transfer to a small plate.
Crack the eggs into a small bowl, and use a fork to beat them with the salt and pepper.
Return the pan to the heat and add the butter. Let it melt, and swirl the pan so that it coats the bottom evenly. Pour the eggs into the pan and quickly shake and swirl the pan so that the eggs cover the entire bottom of the pan. Let the eggs firm up on the bottom, about 30 seconds, then use a rubber spatula to lift the edges of the omelet up so that any uncooked egg on the top runs underneath. Sprinkle half of the eggs with the sauteed spinach and the feta and let it cook for another 30 seconds; the top should be moist but not quite runny (unless you like it runny). Flip the untopped side of the eggs over the filling, and slide it onto a plate. Garnish with fresh chives if desired.