Prepare the ingredients.Remove any thick stems from the spinach. Break large pieces of feta into pea-size crumbles. Place all ingredients within reach of the stove.
Crack the eggs into a medium bowl. Add the water, salt, and two or three turns of black pepper. Whisk for 30 to 45 seconds, until no clear egg-white streaks remain.
Place the skillet over medium heat. Add the spinach to the dry pan and stir for 60 to 90 seconds, until the leaves collapse and turn deep green.
Transfer the spinach to a small plate and spread it out so the steam can escape. Pour away any liquid left in the skillet.
Reduce the heat to medium-low. Add the butter and tilt the skillet until the base has a thin, even coating. Do not let the butter brown.
Pour the egg mixture into the center of the skillet. Leave it untouched for 10 seconds. Pull the cooked edges toward the middle with the spatula, then tilt the pan so loose egg runs into the open spaces. Repeat for 30 to 45 seconds.
Place the spinach over one half of the eggs, leaving a ½-inch border around the edge. Sprinkle the feta evenly over the spinach.
Slide the spatula beneath the empty half and fold it over the filling. Hold it in place for 20 to 30 seconds, then turn off the heat.
Tilt the skillet toward a plate and guide the omelette out with the spatula. Rest it for 1 minute before cutting.
