Cook the pasta in boiling, salted water until tender, then drain, refresh in cold water and drain again.
Place the cooked pasta in a large bowl with the onion and tomatoes. Mix together the lemon juice, vegetable stock and oil, season with salt and pepper and pour over the salad.
Leave to stand for about 10 minutes then mix in the rocket, basil and tuna. Season with salt and pepper if necessary then serve.