Heat 1 tablespoon of the oil in a heavy-based saucepan, add the onion and cook for 5 minutes until softened. Add the garlic, chilli flakes and paprika and cook for a further 30 seconds.
Pour in the wine and bubble for a couple of minutes until reduced by half. Add the tomatoes and sugar and season with salt and pepper, then cook for 20 minutes, adding a little water if necessary.
Meanwhile, cook the potatoes in a saucepan of boiling water for 5 minutes, then drain well. Heat the remaining oil in a large frying pan, add the potatoes in a single layer (you may have to do this in 2 batches) and cook for 10–15 minutes, turning frequently, until golden and crispy all over.
Transfer the potatoes to a serving bowl and serve with the tomato sauce spooned over.