Heat the oven to 425°F and line a rimmed sheet pan with parchment.
Toss the sweet potatoes with 1 tablespoon olive oil, half the salt and the black pepper, spread them out and roast for 10 minutes.
Whisk the remaining olive oil with Dijon and maple syrup, then coat the chicken and green beans with the mixture and remaining salt.
Turn the potatoes, add the chicken and beans in one layer and roast for 18 to 20 minutes, until the thighs reach 165°F and the potatoes are tender.
Rest the chicken for 3 minutes before spooning the sticky pan juices over each portion.