Crack the eggs into a bowl, add the sea salt and chopped onions, and blend together with a wire whisk.
Melt 3 tablespoons of butter in a frying pan, and then add the egg mixture, tipping the pan to make sure the egg mixture covers the bottom of the pan.
Cook on medium heat, until the underside is lightly browned, then add the crumbled feat cheese, spinach, and chicken sausage.
Lift one side of the omelet with a spatula, and fold it in half.
Reduce the heat, and cook until the inside of the omelet is cooked (less than a minute or so).
Use a spatula to slide the omelet onto a plate and serve!
Repeat for a second omelet.