Heat the oil in a pan and add the onion, fry for about 5 minutes until softened. Add the garlic, tomatoes, tomato purée, paprika, chilli powder and sugar and bring to the boil, stirring continuously. Simmer for 10 minutes until pulpy. Set aside for up to 24 hours.
Preheat the oven to 180°C/200°C/gas mark 6/. Take the cubed potatoes and pat dry with kitchen paper.
Spread over a roasting tin and toss the potatoes in the oil, season with salt and pepper. Roast for 40-50 minutes until the potatoes are crisp and golden brown.
Gently reheat the sauce in a saucepan. Tip the potatoes into dishes and spoon over the sauce. Sprinkle with the parsley.
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