Peel and finely chop the onion. Heat the oil in a pot. Sauté diced onion in it. Add tomato paste and sweat. Add tomatoes, chop a little. Bring sauce to the boil and let it boil down for about 10 minutes.
Cook the pasta in boiling salted water according to the instructions on the packet. Drain the tuna, tear apart easily and add to the sauce. Wash the thyme and dab dry. Strip and chop leaves from 4 stems and add to the sauce, except for something to sprinkle.
Add capers and white wine to the sauce. Season with salt, pepper and sugar. Drain the pasta, mix with the sauce, serve and sprinkle with the remaining thyme and garnish.