1. Position rack in lower third of oven; preheat to 450°F. Place a large rimmed baking sheet in the oven to preheat.
2. Combine mustard, thyme, 1 tablespoon oil and 1/4 teaspoon each salt and pepper in a small bowl; spread the mixture evenly on chicken.
3. Toss sweet potatoes and onion in a bowl with the remaining 1 tablespoon oil and 1/4 teaspoon each salt and pepper. Carefully remove the baking sheet from the oven and spread the vegetables on it. Place the chicken on top of the vegetables.
4. Return the pan to the oven and roast, stirring the vegetables once halfway through, until the vegetables are tender and beginning to brown and an instant-read thermometer inserted into a chicken thigh registers 165°F, 30 to 35 minutes.