Sprinkle roast with seasonings. Place onions, carrots and celery in crockpot. Top with meat. Add bay leaf, vinegar and water. (Remove the bay leaf before serving.) Cover pot and cook on low 5 to 7 hours or meat is tender. Remove meat and turn on high. Add cabbage wedges; cover and cook on high 15 to 20 minutes or until cabbage is done.
Meanwhile, melt butter in saucepan. Stir in instant onion and flour. Drain 1 1/2 cups of broth out of cooking pot. Pour broth, horseradish and salt into saucepan. Cook over low heat, stiring constantly, until thickened and smooth. Serve sauce over roast, with vegtables.