Place a rimmed ¼ sheet pan into the oven and preheat to 400 degrees.
Meanwhile, pat the chicken with paper towels to remove any excess moisture.
Drizzle 2 tablespoons of the oil over the chicken and rub to coat.
Season generously with salt and pepper.
Carefully place the chicken, skin-side up, onto the hot sheet pan and roast for 30 minutes.
In a large bowl, combine the onion, garlic, tomatoes, capers, vinegar, sugar, remaining tablespoon of oil, ½ teaspoon salt and a generous pinch of pepper and mix well.
Scatter the tomato mixture around the chicken and roast for another 30 minutes.
Cool for 5 minutes before serving, garnishing with parsley, if desired.