For chicken, arrange chicken, capsicum, onion, garlic and thyme in a large roasting pan.
For marinade, whisk vinegar, oil and brown sugar in a bowl until sugar has dissolved. Drizzle half the mixture over the chicken and a little over the vegetables.
Bake for 15 minutes and add cherry tomatoes. Drizzle a few tablespoons of marinade over chicken and bake for a further 20-25 minutes. Retain pan juices.
For mash, boil potatoes in a saucepan of water until tender. Drain and return to saucepan.
Mash until almost smooth. Add sour cream, butter and most of the spring onion. Season with salt and pepper and sprinkle extra spring onion on top.
For sauce, place remaining marinade in a saucepan with pan juices. Stir over low heat until thickened.
To serve, arrange vegetables on a plate with chicken on top. Drizzle sauce over chicken and serve with champs mash and extra sauce in a gravy boat on the side.