In 5- to 6-quart slow cooker, place onion. If beef roast comes in netting or is tied, remove netting or strings. Place beef on onion. Place potatoes and garlic around beef. In small bowl, mix 1 1/4 cups of the broth and the dry soup mix; pour over beef. (Refrigerate remaining broth.)
Cover; cook on Low heat setting 9 to 10 hours.
Remove beef and vegetables from cooker; place on serving platter. Cover to keep warm.
In small bowl, mix remaining 1/2 cup broth and the flour; gradually stir into juices in cooker. Increase heat setting to High. Cover; cook about 15 minutes, stirring occasionally, until sauce has thickened. Serve sauce over beef and vegetables.