Preheat your oven to 425°F (220°C).
In a small bowl, whisk together lemon juice, olive oil, minced garlic, chopped thyme, salt, and black pepper to form a marinade.
Place the chicken breast in a shallow dish and pour half of the marinade over it. Let it marinate for at least 15 minutes while you prepare the vegetables.
Peel (if desired) and cut the sweet potato into 1/2-inch cubes, and trim the green beans by snapping off the ends.
Toss the sweet potato cubes and green beans in the remaining marinade, ensuring they are evenly coated.
Arrange the chicken, sweet potatoes, and green beans on a baking sheet lined with parchment paper, keeping the vegetables in a single layer.
Roast in the preheated oven for about 20-25 minutes, or until the chicken is cooked through (internal temperature 165°F), the sweet potatoes are tender and slightly crispy, and the green beans have a slight char.
Remove from the oven and let rest for a few minutes before serving to allow the flavors to settle.