Finely chop the onion and slice the leeks.
Finely shred the cabbage, rinse and drain.
Heat the oil in a pan, then gently cook the leek and onion for 5 minutes, until soft.
Meanwhile, peel the potatoes, then dice. Mince the garlic. Add these to the sautéed onion, stir for 1 or 2 minutes, then pour in the stock.
Bring to the boil and simmer, under cover on a low heat, for 15-20 minutes. Then, using a hand-held blender, process until you a get coarse-textured soup. Add the cabbage slices. Put the lid on and cook for about 3 to 5 minutes. Season with salt and pepper to your taste.
Meanwhile, dry-fry the Prosciutto in a frying pan until crispy.
Serve the soup, adding a dollop of sour cream if you like, and a slice of crispy Prosciutto.