Preheat the oven to 375F/190C.
Put the chicken thighs into a Ziploc bag, then add the balsamic vinegar, honey, mustard, herbs, and cornstarch (corn flour). Close the top of the bag and smoosh around until the chicken is well covered in the marinade. Place the bag on a plate and store in the fridge while you assemble the rest of the dish.
Line a baking tray with baking paper, then scatter the brussels sprouts, mushrooms and onions over the tray. Drizzle over a little olive oil, then toss with your hands or a big spoon.
Tuck the chicken from the Ziploc bag in among the vegetables, then pour the rest of the marinade over the chicken. Dot the top of the chicken with little bits of the butter.
Sprinkle with plenty of salt and pepper, then roast for 35 minutes, or until the chicken is cooked through and the vegetables are tender and starting to caramelise in parts.
Serve with toasted crusty bread, rice or potatoes.
