Preheat the oven to 220C, 200C fan, gas mark 6. Strip half the thyme leaves from the stalks, add the leaves to a small bowl along with the garlic, balsamic, rose wine, olive oil and seasoning. Whisk with a fork to combine.
Place the chicken in a high-sided roasting dish. Push half the orange and lemon slices into the chicken cavity, along with the remaining whole thyme sprigs. Place the remaining orange and lemon slices and onion wedges around the chicken in the roasting dish.
Pour the marinade over the whole chicken. Turn the chicken on its side, sitting on one leg propped up with the fruit if necessary. This helps cook the bird more evenly and ensures juicy chicken breasts. Roast for 20 minutes. Carefully turn the chicken over onto its other side, sitting on the other leg. Baste with the juices, and return to the oven for another 20 minutes. Finally, turn the chicken on its back, breast side down, baste once more and roast for a final 20 minutes. Remove from the oven, baste in the juices and let rest for 20 minutes under a foil tent.
While the chicken is resting, bring a saucepan of water to the boil, then cook the carrots for 8-10 minutes or until just tender. Add the peas for the final 2 minutes.
Serve the rosé roast chicken on a platter with the roasted orange, lemon, and onion, with the vegetables on the side.