Sprinkle roast with seasonings. Place onions, carrots and celery in crockpot. Top with meat. Add bay leaf, vinegar and water. Cover and cook on LOW 5-6 hours, or until meat is tender. Remove meat and turn on HIGH. Add cabbage wedges, cover and cook on HIGH 15-20 minutes, or until cabbage is done. Meanwhile, melt butter in saucepan. Stir in instant onion and flour. Drain 1 1/2 cup of broth out of cooking pot. Pour broth, horseradish and salt into saucepan. Cook over low heat, stirring constantly until thickened and smooth. Serve sauce over roast with the vegetables.