Preheat oven to 400°F. Line a sheet pan with parchment paper.
Whisk together the balsamic vinegar, honey, garlic, Dijon, thyme, salt, and pepper in a small bowl.
Toss the Brussels sprouts and onion with 2 tablespoons olive oil, salt, and pepper. Spread on the sheet pan.
Using the same bowl, add the chicken, season with salt and pepper, drizzle with olive oil, and add half of the balsamic glaze. Toss to coat, then arrange on the sheet pan.
Roast for 15 minutes. Remove pan, flip Brussels sprouts, and drizzle everything with the remaining glaze.
Roast another 10-15 minutes until chicken reaches 165°F and Brussels sprouts are golden and crispy.
Let rest 5 minutes before serving. Drizzle any pan juices over everything.