Separate garlic bulb into cloves. Mince two cloves; place in a small bowl. Add the butter, thyme, salt and pepper. In a large bowl, combine the sweet potatoes, brussels sprouts, onions and remaining garlic cloves. Drizzle with butter mixture; toss to coat.
Transfer to a greased 13-in. x 9-in. baking dish. Cover and bake at 350° for 30 minutes. Uncover, bake 40-45 minutes longer or until vegetables are tender. Stir before serving.