On a heavy bottomed pan on a medium heat, heat the olive oil. Add the potatoes to the pan and cover loosely with a saucepan lid. Stir occasionally to avoid them sticking.
When the potatoes are slightly cooked (about 3 to 5 minutes depending on the size of the chunks), add the onion. Pan fry on a medium heat for about 3 minutes.
Add the chorizo and cook lightly for about a minute to release the oils.
Mix the eggs, milk and seasoning. Preheat the grill at a medium setting.
Make sure the potatoe, onion and chorizo mix is spread even across the pan. Add the egg mix and sprinkle the cheddar evenly across the top. Cook on the pan for one minute.
Remove from the heat and put under the grill. Cook for roughly 5 minutes or until the egg is cooked through.
Remove and allow to cool slightly before eating. If storing, allow the Frittata to cool completely, and cover tightly with clingfilm, before storing in the fridge. The Frittata can be eaten cold or hot (4 minutes in a 800W microwave) as preferred.