Preheat your oven to 425°F (220°C).
In a small bowl, whisk together olive oil, lemon juice, chopped thyme, rosemary, minced garlic, salt, and pepper.
Place the chicken thighs in a shallow dish and pour half of the lemon-herb marinade over them. Let marinate for at least 15 minutes.
Meanwhile, peel (if desired) and dice the sweet potato into 1-inch cubes. Trim the ends of the green beans.
On a baking sheet, toss the sweet potato cubes with a little olive oil, salt, and pepper. Roast in the preheated oven for 10 minutes.
After 10 minutes, add the green beans to the baking sheet with sweet potatoes, drizzle with the remaining marinade, and toss to combine.
Place the marinated chicken thighs on a separate baking tray. Roast in the oven alongside the vegetables for 20-25 minutes or until the chicken reaches an internal temperature of 165°F and the vegetables are tender.
If desired, broil for an additional 2-3 minutes to crisp up the chicken's exterior.
Remove from oven, plate the chicken with a generous serving of roasted sweet potatoes and green beans, and serve immediately.