In 5- to 6-quart slow cooker, place onion. Place beef on onion. Place carrots, 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.