Heat the oil in a pan and fry the onion for about 5 mins until softened. Add the garlic, chopped tomatoes, tomato purée, sweet paprika, chilli powder, sugar and a pinch of salt, then bring to the boil, stirring occasionally. Lower to a simmer and cook for 10 mins until pulpy. Can be kept chilled for up to 24 hrs.
Heat oven to 200C/180C fan/gas 6. Pat the potatoes dry with kitchen paper, then tip into a roasting tin and toss in the olive oil and some seasoning. Roast for 40-50 mins until crisp and golden. Tip the potatoes into serving dishes and spoon over the tomato sauce. Sprinkle with some fresh parsley to serve.