Sprinkle meat with seasonings. Place onions, carrots, and celery in crockpot. Top with meat. Add bay leaf, vinegar and water. Cover pot and cook on low 5-7 hours or until meat is tender. Remove meat, 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 cups 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 vegetables.