Drain the tuna and pluck it into pieces. Peel onion and cut into rings. Drain the olives and cut into rings. Clean and wash the rocket. Stir ketchup and 1 tablespoon of oil into a third of pasta.
Season with salt, pepper and sugar. Fold in tuna, onion, olives and rocket. Arrange the tuna salad on a plate, sprinkle with parmesan slices.