Prepare the pasta in boiling salted water according to the instructions on the packet. Wash the marjoram, dab dry and finely chop half of the leaves. Heat margarine and sauté onion cubes in it.
Add milk, sour cream and half of the cheese and simmer for 2-3 minutes. Puree and season to taste with salt and pepper. Add zucchini cubes and simmer for another 2-3 minutes. Add olives and chopped marjoram at the end.
Thicken the sauce with some sauce thickener if necessary. Drain spaghetti, serve with the sauce and garnish with marjoram leaves and cheese cubes.