Put onions and snap peas in a bowl and carrots in another. Toss each with half the peanut oil and sesame seeds. Add carrots to pan of chicken when chicken has about 10 minutes left to cook.
Tip juices out of chicken into pan, then transfer chicken to a platter. Increase oven to 500°, toss snap peas and onions with carrots and pan juices, and roast until tender-crisp, about 5 minutes. Toss vegetables with sesame oil.