Heat an 8-inch non-stick pan over medium heat on the stove. Add oil or butter, and as soon as it is hot, add the leeks and fry on medium heat until lightly browned.
Add the garlic and fry until fragrant, a minute or so.
Add the spinach and salt. Saute until wilted and soft, for a minute or so.
Beat two eggs and add a couple of pinches of salt. If beating eggs before cooking the veggies, do not add salt until right before the eggs go into the pan.
When the eggs start to turn into an omelette (meaning that the base is coming together but the top is still gooey), spread the cooked spinach and leeks on the top evenly. Sprinkle over with coarsely ground pepper and paprika or cayenne, depending on how much heat you like in your omelette. Turn the heat back up to medium and cook for a minute or more, depending on how cooked you like your eggs.
If you have a deft hand, you can try to flip the omelette, or you can turn the omelette over itself and make a half moon. I prefer it this way. Then flip this half moon to give the other side the chance to brown a little.
When it's browned to your liking, turn the heat off, take the pan off the hot stove, and cover it with a lid for a minute. The residual steam will make sure that the omelette is cooked through.