Boil the pasta in salted water and drain it in a deep bowl. Mix with butter while still warm. Wash, clean and grate the tomatoes. Heat 2 tablespoons of olive oil in a pan and add the tomato sauce and finely chopped olives.
Allow 10-15 minutes for the sauce to fry a bit, then remove the pan. Gradually add the mayonnaise, mix and season with tarragon and salt. Evenly distribute the spaghetti into 2 plates.
Drain the marinade from the canned fish. Serve a plate of spaghetti, covered with sauce and tuna on top.