Cut slits in roast, using a sharp knife; push a garlic slice into each slit. Sprinkle roast with salts and pepper; lightly dredge in flour, patting off excess flour.
Brown roast on all sides in hot oil over medium-high heat in a large Dutch oven. Stir together coffee, soup, and Worcestershire sauce; pour over roast. Top with onion. Reduce heat, cover, and simmer 3 hours and 40 minutes or until tender. Transfer roast to a serving platter, reserving drippings in Dutch oven; keep roast warm.
Combine cornstarch and 3 tablespoons water; stir into drippings. Bring mixture to a boil, and cook, stirring constantly, 1 minute or until thickened. Pour gravy over roast.