Preheat the oven to 180C/350F/Gas 4.
Rub the chicken with olive oil and season well with salt and freshly ground black pepper. Heat a griddle pan and griddle the chicken for 4-5 minutes on each side.
Transfer to a baking sheet and bake in the oven for 12 minutes, or until the chicken is completely cooked through.
For the sauce, heat the olive oil in a small saucepan, add the onion and gently fry for 3-4 minutes. Add the garlic and fry for one minute.
Add the tomatoes and cook for a further minute. Add the vinegar, sugar, tomato ketchup and tomato purée and simmer for five minutes. Stir in the basil, reserving some to garnish, and season, to taste, with salt and freshly ground black pepper.
To serve, spoon the tomato sauce onto a serving plate and top with the griddled chicken. Garnish with the reserved basil leaves.