Cook the spaghetti in salted water according to the instructions on the packet until al dente.
In the meantime, roast the onion in a pan with a little oil. Cut the zucchini into 1 cm cubes. Also add to the pan and let roast briefly. Pour in the vegetable stock. Stir in tomato paste, garlic and sugar and season with a little salt and pepper. Simmer for about 10 minutes until firm to the bite.
Dip the tomatoes briefly in hot water, rinse with cold water and peel off the skin. Then quarter, remove the stalk and seeds and dice. Drain the tuna.