Season the chicken breasts with salt and pepper. Spray a large, deep non-stick flameproof frying pan or sauté pan with oil and place over a high heat. Cook the chicken on each side for 3 minutes or until lightly browned. Transfer to a plate.
Reduce the heat to low, spray a little more cooking oil into the pan and cook the onion for 4-5 minutes, stirring until softened and lightly browned. Add the garlic and cook for a few seconds.
Pour in the tomatoes. Stir in the tomato purée, oregano, olives and 300ml/10fl oz cold water. Bring to the boil and cook for 5 minutes, stirring regularly. Reduce the heat to a gentle simmer and add the chicken. Cook for 10 minutes, stirring occasionally, until the chicken is tender and cooked through. Season to taste.
Preheat the grill to its hottest setting. Top the chicken and sauce with the sliced mozzarella. Sprinkle with ground black pepper. Place under the hot grill and cook for 2-3 minutes or until the cheese melts.
For the butter bean mash, spray a small non-stick saucepan with cooking oil and place over a medium heat. Add the onion and cook for 3 minutes. Add the garlic and cook for 1 further minute.
Put the beans in a food processor with 100ml/3½fl oz water, a good pinch of salt and pepper. Add the cooked onion and garlic and blend to a thick purée. Spoon back in to the saucepan and heat, stirring constantly until hot. Add a good squeeze of lemon and season to taste.
Divide the butterbean mash between 4 plates and add the chicken and sauce. Serve with freshly cooked vegetables or salad.