Peel and finely dice the onions and garlic. Wash the tomatoes, dab dry and cut into pieces. Quarter peppers, remove seeds, wash and cut into cubes. Wash and dice the zucchini.
Pluck marjoram leaves from the stalks and chop finely. Heat the olive oil in a pan and fry the onions and garlic for about 10 minutes over a low heat. Add tomatoes, peppers and zucchini and fry for about 2 minutes more.
Deglaze with vegetable stock, bring to the boil, cover and braise over medium heat for about 10 minutes. Meanwhile cook spaghetti in boiling salted water for about 10 minutes. Drain and let drain. Stir marjoram into the sauce and season with salt, pepper and sugar.
Arrange noodles and sauce on plates and serve garnished with marjoram.