Cook the pasta in plenty of boiling salted water according to the instructions on the packet until al dente. Peel and finely dice the onion. Heat the oil in a pot and sauté the onion. Add tomatoes and chop a little.
Season with salt and sugar and bring to the boil. Wash the chilli pepper, cut lengthwise and remove the seeds. Cut the pod into fine rings. Drain olives and cut into rings. Wash parsley, dab dry, pluck leaves from the stalks and cut into strips.
Cut the dried tomatoes into cubes. Pluck tuna into large pieces. Stir chilli, olives, parsley, capers, dried tomatoes and tuna into the tomato sauce. Drain the pasta and arrange on plates with the sauce.
Garnish with parsley.