Trim fat from lamb. Cut 1-inch slits in meat; insert half of garlic slices into slits, reserving remaining garlic. Rub meat with 1 teaspoon dried rosemary and 1/2 teaspoon pepper; place on a rack in a large roasting pan coated with cooking spray. Insert meat thermometer into thickest part of meat, if desired. Add potato to pan. Bake at 325° for 1 hour.
Remove stems from mushrooms; reserve stems for another use. Combine mushroom caps, tomato, eggplant, red pepper, yellow pepper, and onion; add to roasting pan.
Combine remaining garlic, 1 teaspoon dried rosemary, 1/2 teaspoon pepper, broth, vinegar, and oil; stir well. Pour broth mixture over vegetables in roasting pan. Bake at 325° for 50 minutes or until thermometer registers 150°. Let stand 10 minutes before serving. Garnish with rosemary sprigs, if desired.