Cook the spaghetti in salted water according to package instructions. Chop the onion and garlic and sauté in the oil. Add the tomato pieces and simmer for 10 minutes.
Chop the parsley. Drain the tuna, chop it a bit and add it to the sauce. Add capers, cream and thyme to the sauce. Bring to a boil and season with salt, pepper and sugar.
Mix in the parsley. Drain the pasta, reserving 1/8 l of the cooking water. Add the pasta to the sauce and mix. Add a little cooking water if necessary.