Cook onion in butter in Dutch oven until soft and lightly colored. Blend in flour. Add water and vinegar and cook, stirring constantly, until mixture comes to a boil. Blend in salt, sugar and pepper. Stir in cabbage and sausage. Cover and simmer about 20 minutes, or until cabbage is tender. Stir occasionally. This may also be done in a crockpot on low for 4 to 6 hours.