Thinly slice the leek and set aside. Remove the skin from the chorizo, then cut it into small cubes.
Heat a small frying pan over medium heat and add a little oil if needed. Add the chorizo and leek, then cook gently for a few minutes until the chorizo releases its oils and the leek softens.
In a bowl, lightly whisk the eggs. Stir in the oregano and grated cheese, if using.
Pour the egg mixture over the leek and chorizo, tilting the pan if needed so it spreads evenly.
Cook for a couple of minutes, then cover the pan with a lid to help the top set.
Once the omelette is fully set, remove from the heat.
If you like, scatter a little extra cheese on top and place the pan briefly under a hot grill to brown.
Serve warm, topped with fresh herbs of your choice or a few cress leaves.