Basically any vegetable can be used for this soup. You just have to make sure that the harder varieties (root vegetables) are put into the pot about 10 minutes before the rest of the vegetables.
Dice the onion. Remove the greens from the leeks, wash the leeks and cut into slices. Wash cauliflower and cut into florets. Peel the celery and kohlrabi and cut into cubes. Cut the carrots into slices about 1/2 cm thick. Cut tomatoes into wedges. Fry the onion, carrots and celery cubes in oil or butter. Add the stock, season with salt, pepper and crushed garlic. Let cook for about 10 minutes. Add the rest of the vegetables and cook for another 15 minutes.
Break the spaghetti once or twice, depending on the length, and add to the soup. Let simmer until tender. Season to taste with salt and pepper. You can also add any herbs.