Put the chicken onto a board. Slice he breast in half through the middle to make two thinner pieces. Cover with clingfilm and bash out to make an even thickness. Season and coat in plain flour.
Heat the oil in a large frying pan. Add the breasts and fry over a high heat to seal the meat. Set aside. Heat 1 tbsp of oil in a frying pan. Add the onions and fry for 3 to 4 minutes.
Add the garlic and fry for 30 seconds. Add the garlic and fry for 30 seconds. Add the fresh and tinned tomatoes. Bring up to the boil, then simmer for 15 minutes or until you have a thick sauce. Add the tomato paste.
Spoon the tomato sauce into a shallow ovenproof dish, Sprinkle over the fresh basil. Arrange the chicken on top. Put one slice of mozzarella on top of each piece of chicken.
Bake in the oven for 20 minutes until bubbling and the chicken is cooked through.