Coat overproof 6-cup-dish with cooking spray. Cut Sweet Potato into 5-mm slices.
Combine with garlic, onion and stock in a pan. Cover and simmer until sweet potato is tender.
Place sweet potato, peas and broccoli in prepared dish. Melt Butter in pan and stir in flour. Stir over heat until bubbling. Remove from heat and gradually stir in combined milk and extra stock. Stir over heat until boiled. Stir in herbs.
Pour Sauce over sweet potato. Top with breadcrumbs.