Place a rack in the upper-middle position and heat the oven to 425°F (220°C). Line a large rimmed sheet pan with parchment paper.
Toss the potatoes and onion wedges with 1 tablespoon of the vegetable oil, half the salt and the pepper on the sheet pan. Spread them out, cut sides down, and roast for 15 minutes.
Meanwhile, pat the chicken thighs very dry. Rub them with the olive oil, then season all over with the salt, pepper, garlic powder and thyme. Toss the Brussels sprouts with the remaining vegetable oil and salt.
Pull out the pan, push the potatoes and onions to the edges, add the Brussels sprouts, and nestle the chicken thighs skin side up in the center. Roast for 25 minutes, until the skin is golden and starting to crisp.
While the chicken roasts, combine the balsamic vinegar, honey, Dijon, garlic, rosemary and salt in a small saucepan. Simmer over medium heat, stirring often, for 6 to 8 minutes, until reduced by about half and thick enough to coat a spoon. It thickens more as it cools.
Brush about two-thirds of the glaze over the chicken, scatter the cherry tomatoes over the pan, and roast for 8 to 10 minutes more, until the glaze is sticky and the thighs reach at least 175°F (79°C) in the thickest part, away from the bone.
Rest the chicken for 5 minutes. Drizzle the remaining glaze over the chicken and vegetables, sprinkle with fresh rosemary, and serve straight from the pan.