Preheat oven to 400°F. In a large baking pan, toss sweet potato cubes in 2 tablespoons oil and arrange in single layer. Bake for 10 minutes.
Meanwhile, toss green beans with remaining oil and thyme. Rub chicken with garlic and pepper and top each portion with a lemon slice.
Remove baking pan from oven; with a spatula, push sweet potatoes to one end of pan, place green beans on other end and arrange chicken in the middle. Bake for an additional 25 minutes or until chicken reaches internal temperature of 165°F.