Place onions in a slow cooker. Rub brisket with pepper, place over onion. Top with mushrooms. In a bowl, combine the broth, chili sauce, brown sugar, and garlic, pour over brisket. Cover and cook on low 8-9 hours or until meat is tender.
Remove brisket and keep warm. In a small bowl, combine flour and water until smooth, stir into cooking juices. Cover and cook on high for 30 minutes or until thickened. Slice brisket, serve with gravy.
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