This is easy. Slice all the ingredients and then start to layer them in an oven proof dish. Start with a layer of potatoes, then add some onion, garlic and chorizo, Cover this with another layer of potato and so on until you have several layers of the dish. If you want you can add some cheddar to each layer, also you will need to season the layers as you go along.
Once this is complete add stock so it comes almost all the way up and over the potatoes. Place in a hot oven (200C) and bake for about 30mins or until the potatoes are tender.