Trim any excess fat from roast. Cut deep slits in roast, using a paring knife; insert garlic slices. Stir together pepper and thyme; rub over all sides of roast, and set aside.
Sauté onion in a nonstick skillet coated with cooking spray over medium heat until tender. Stir in chicken broth and next 7 ingredients; bring to a boil, stirring constantly. Remove from heat.
Place roast in a lightly greased large Dutch oven; pour sauce over roast.
Bake, covered, at 350° for 1 hour and 20 minutes or until a meat thermometer registers 170°. Let stand 15 minutes. Slice and serve with sauce; garnish, if desired.