In a medium bowl, whisk together the liquid egg whites, whole egg, and Greek yogurt until the mixture is completely smooth and slightly frothy.
Heat the olive oil in a non-stick skillet over medium heat and sauté the spinach until just wilted, about 1-2 minutes.
Remove the spinach from the pan and set aside, then wipe the skillet clean if necessary.
Pour the egg mixture into the skillet, tilting the pan to ensure the bottom is evenly covered.
As the edges set, gently lift them with a spatula and let the uncooked egg flow underneath until the omelet is mostly firm.
Sprinkle the sautéed spinach and crumbled feta cheese over one half of the omelet.
Fold the omelet in half, cover the pan for 30 seconds to melt the cheese, and slide onto a plate.
Season with sea salt and black pepper, then top with sliced avocado before serving.