In 5 quart pot, sauté onion in butter until golden. In bowl soak sauerkraut in water, drain and chop. Add to onions and cook until done, adding water if mixture gets too dry. Add tomato puree. Sauté 5 to 7 minutes. Add potatoes, bulgur, and stock; cook 15 minutes. Season to taste with salt and pepper. Garnish with sliced green pepper and parsley.