Preheat the oven to 200 degrees celsius (fan).
Using a good sharp knife, dice the chorizo into thumbnail sized chunks and place into a medium sized frying pan. Gently crisp up the chorizo over a low heat. It should release a lovely orangey-red flavourful oil, but try not to burn the chorizo. When it is crisp and cooked through, set to one side, leaving the oil in the pan.
Peel the potatoes and chop into chunks approximately 3cm square. You can make these bigger or smaller if you like, but will need to alter the cooking time accordingly. Toss in olive oil, salt and pepper and roast in the oven for approximately 30 minutes.
Finely slice the red onion and add to the frying pan along with a pinch of salt. Cook the onion gently over a low heat until it is soft and translucent. Whilst it is cooking, roughly chop the tomatoes (if using fresh tomatoes).
Finely slice the garlic cloves and add to the onion along with the smoked paprika. Cook for a minute over a low heat before adding the tomatoes, red wine or sherry vinegar, and a pinch of caster sugar. Turn up to a medium heat - if using fresh tomatoes, after a few minutes the tomatoes will begin to break down and release their juices. When the sauce begins to bubble, turn down the heat to take it to a gentle simmer and return the chorizo to the pan, along with a handful of olives and another pinch of salt and twist of black pepper. Give it a good stir every couple of minutes. Roughly chop the flat leaf parsley and set to one side.
Whilst the sauce is simmering, you can cook the fish. Place about a teaspoon of olive oil in a non-stick frying pan and heat over a medium heat. The pan should be large enough to fit both pieces of fish in at the same time, with plenty of space around each. Season each side of the fish with salt and pepper and place in the pan, skin side down.
Turn the heat down to low and let the fish cook gently. The skin should be crisping up and you will start to see the flesh of the fish change from slightly translucent to white, cooking from the bottom upwards. When the fish has changed colour almost to the top, flip the fillet over, to cook from the other side. All of this should be done over a gentle heat. After a minute or two, the top of the fish should be cooked and starting to get some colour on the top side. Turn it back over so that it is skin-side down and add a knob of butter to the pan. When the butter is melted, repeatedly spoon it over the fish several times.
I can’t give you cooking times for the fish because it will vary dramatically depending upon the thickness of your fish fillet or steak. The best way to check whether fish is cooked is to either check it by eye (if you cut into the centre, the fish should be opaque and white), or by using a temperature probe. Fish is cooked when it reaches 50 degrees celsius. The temperature will continue to rise by several degrees once you remove it from the heat source.
When the fish is cooked, take it off the heat and set aside to rest. By now, the sauce should have thickened and be more of a stew-like consistency. Check the seasoning, adding more salt and pepper if needed. If you feel it needs a lift, add a squeeze of lemon juice. Stir in half of the chopped flat-leaf parsley.
When your potatoes are golden, crispy and cooked through, they are ready to serve. I like to place half the sauce in the middle of the plate, with the fish on top and the potatoes scattered around the edge. Sprinkle with the remaining flat-leaf parsley and serve.