In a small saucepan, bring broth to a boil; remove from heat. Add tea bags; steep, covered, 3-5 minutes according to taste. Discard tea bags. Meanwhile, combine vegetables in a 6-qt. slow cooker., In a large skillet, heat oil over medium-high heat; brown roast on all sides. Place over vegetables; pour tea-steeped broth over top. Sprinkle roast with salt and pepper; top with lemon slices. Cook, covered, on low until beef and vegetables are tender, 7-9 hours., Discard lemon slices. Remove roast and vegetables from slow cooker; keep warm., Transfer cooking juices to a saucepan; skim off fat. Bring juices to a boil. In a small bowl, mix cornstarch and water until smooth; stir into juices. Return to a boil, stirring constantly; cook and stir until thickened, 1-2 minutes. Serve with roast and vegetables.