About 2 1/2 hours before serving, heat dutch oven until very hot. Add butter, then short ribs. Cook over high heat until ribs are very well browned on all sides, about 20 minutes.
To ribs, add onion slices; push them down into fat around ribs, then cook a minute or so, until browned. Now add salt, pepper, 1 teaspoons sugar, allspice, bay leaves, and 1 cup water. Cover and simmer 1 1/2 hours or until meat is very tender.
Remove ribs to heated platter and keep warm. Skim about 1/4 fat from surface of gravy; Also remove allspice and bay leaves. Stir 1/4 cup water into flour until smooth; slowly add, stirring, to gravy; cook until thickened. Stir in heavy cream, 1 teaspoons sugar, and enough water to make gravy as you like it, bring to a boil; then pour over ribs and serve.