Prepare ahead by heating the oil in a pan, add the onion and fry for about 5 minutes until softened. Add the garlic, tomatoes, tomato purée, paprika, chilli powder, sugar and salt and bring to the boil, stirring. Simmer for 10 minutes until pulpy. Set aside for up to 24 hours.
To serve, preheat the oven to 200C/gas 6/fan oven 180C. Cut the potatoes into small cubes and pat dry with kitchen paper. Spread over a roasting tin and toss in the oil, then season. Roast for 40-50 minutes, until the potatoes are crisp and golden.
Tip the potatoes into dishes and spoon over the reheated sauce. Sprinkle with the parsley. Serve with cocktail sticks.