Thinly slice the leek.
Heat a spoonful of oil in frying pan, and sauté the leek for 5 minutes. Add the lardons and cook until crispy. Transfer half of the leek & bacon mixture to a plate.
Lightly whisk two eggs in a small bowl. Pour this into the pan, allowing to spread out evenly.
When the surface has almost set, scatter some cheese on top if you like (or add baby spinach, fresh herbs for a dairy-free version).
Fold in half and cook for another minute or so, then slide it to a plate.
Keep it warm whilst you make another omelette with the remaining bacon/leek mixture and eggs.