Soak the porcini mushrooms in 300 ml water for approx. 30 minutes. Drain, collect the soaking water, roughly chop the porcini mushrooms. Finely chop the onion and garlic. Wash the vegetables and peel the potatoes. Cut the cabbage into strips and the potatoes into bite-sized, thin slices. Halve the leek lengthways and cut crossways into thin strips, slice the mushrooms.
Heat the butter and oil in a pan and fry the onion until translucent. Add the garlic, cabbage and potatoes, season with salt and stir-fry for 3 minutes. Pour the soup over the vegetables, season with 1 clove, 1 bay leaf, nutmeg and pepper. Simmer for 8 minutes, add the leek, mushrooms and soaking water. Simmer the soup for a further 6 minutes.
Mix the sour cream well with the potato starch and thicken the soup a little. Flavour with salt, pepper and nutmeg.