Heat the oil in a pan and gently fry the onion for 15 minutes until really soft.. Add the garlic and cook for a minute more then add the chilli and tomatoes. Stir in the pimentón, vinegar and sugar, season well and simmer for 20-30 minutes until you have a thick, chunky sauce. Set aside.
Heat a deep pan with olive to 100°C. Fry the potatoes for 10 minutes - they should not be coloured at all at this point. Remove from the pan with a slotted spoon and drain on kitchen paper. Leave to cool completely.
Heat the oil to 160°C. Fry the potatoes for 4-5 minutes until they are tender and just starting to turn golden. Drain on kitchen paper and leave to cool.
When ready to serve, reheat the salsa brava. heat the oil to 200°C and fry the potatoes for 2-3 minutes until really golden and very crispy. Drain and season with salt. Spoon over the salsa brava. You can serve this with store bought or homemade aioli.