Fry the mushrooms in the hot butter on the highest setting and simmer until all the water has evaporated. Then add the onions and peppers and cook on a low heat for 5 minutes. Now add zucchini and carrots and sauté for 5 minutes. Add the curry, paprika powder, chili powder, salt and pepper and stir well. Now add the vegetable stock and simmer for another 5 minutes. Finally add the leek and sour cream and finish cooking for about 10 minutes on a low heat. If you like, you can add a little flour mixed with water to make the sauce a little thicker.