In a skillet, brown roast on all sides in hot oil; drain. Place onion and sweet potatoes in slow cooker; top with roast. Combine broth and seasonings; pour over all. Cover and cook on low setting 7 to 8 hours or on high setting 4 to 5 hours.
Place roast on a serving platter, surrounded with vegetables; keep warm. Combine cornstarch and water in a small saucepan; add one cup of juices from slow cooker. Cook and stir over medium heat until thickened and bubbly continue cooking and stirring 2 more minutes. Serve gravy with roast.