Preheat the oven to 200°C.
Spatchcock the chicken by cutting out the backbone with scissors and flattening it on a roasting tray with red onions.
Spread Dijon mustard and butter over the chicken skin.
Sauté chopped onion, sage, lemon zest, and juice, then stir in dried onion flakes.
Pour the onion mixture over the chicken and roast for 40-45 minutes until golden and cooked through.
Rest the chicken for 10 minutes after roasting.
Preheat the oven to 150°C for the tomatoes.
Roast seasoned tomatoes with thyme and olive oil for 1-2 hours until dried out.
Cool the tomatoes and increase the oven temperature to 220°C.
Bake seasoned ciabatta pieces with olive oil for 5-7 minutes until golden.
Sauté the shallot in oil until softened.
Purée two-thirds of the tomatoes with vinegar and extra virgin olive oil.
Soak the ciabatta croutons in half of the tomato purée for five minutes.
Mix the remaining purée with whole tomatoes, shallots, capers, peppers, burrata, and basil.
Carve the chicken and serves alongside the salad.