For the chicken, place the chicken breast between two sheets of cling film and flatten to 1cm/½in thick with a rolling pin. Discard the cling film.
Drizzle the chicken breast in olive oil, sprinkle with chopped parsley and rosemary and season with salt and freshly ground black pepper. Heat a griddle pan until smoking hot, add the chicken and fry for 2-3 minutes on each side, or until completely cooked through.
For the tomatoes, heat the olive oil in a saucepan, add the onion and fry gently until softened but not coloured. Add the garlic and fry for one minute. Add the chopped tomatoes, salt and freshly ground black pepper, rosemary and parsley and simmer for three minutes.
To serve, spoon the tomatoes onto a serving plate and top with the chicken breast.
