Step 1

Position rack in lower third of oven; preheat to 450 degrees F. Place a large rimmed baking sheet in the oven to preheat.

Step 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.

Step 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.

Step 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 degrees F, 30 to 35 minutes.