Place all brine ingredients into a large saucepan with 2 litres water. Bring to a boil, then remove from heat and let cool completely.
Submerge chicken in cooled brine. Cover and refrigerate overnight.
Remove chicken from brine 30 minutes before roasting to come to room temperature.
Preheat oven to 240°C / 475°F / Gas Mark 9.
Place chicken in a roasting pan and rub with butter. Immediately reduce oven temperature to 200°C / 400°F / Gas Mark 6. Roast for 1 hour and 20 minutes or until juices run clear.
While chicken roasts, place tomatoes in a bowl. Prick each with the tip of a knife. Season with salt, pepper, and a drizzle of olive oil.
Transfer tomatoes to a baking dish and roast in the same oven for 40 minutes, until blistered but not burnt. Set aside in a mixing bowl.
Once the chicken is cooked, let it rest for 15 minutes. Pour pan juices over roasted tomatoes.
Add Baby Gem lettuce, aioli, and lemon juice to the tomato bowl. Toss gently to coat. Adjust seasoning.
Place salad on a large warm platter. Set roast chicken on top and garnish with chopped parsley. Serve warm.