Preheat the oven to gas mark 4, 180°C. Cut the chicken breast in half lengthways straight down the middle. Place each half between a sheet of clingfilm large enough to allow plenty of room for the chicken to expand. Hammer each one with a cook's mallet or a rolling pin until it is around 5mm thick. Don't be too rough or it will tear.
Using three bowls or plates, put the flour in one and season well with sea salt and black pepper, put the beaten egg into another and the mix the parmesan cheese and breadcrumbs in the last one. Coat each piece of chicken breast in flour, then dip in the egg and then the parmesan and breadcrumb mix. Go back to the egg and dip again then back into the parmesan and breadcrumbs once more. That will ensure an even coating.
Put the oil into a hot frying pan and, when hot, add the chicken. Cook until it is golden on both sides (roughly 5 minutes per side), then remove and set aside whilst you concentrate on the sauce.
Whilst the chicken is cooking put the olive oil into a saucepan. Add the onion and cook until the it has softened (about one side of cooking the chicken!). Add the garlic and fry for a minute or so, then add the tomato purée, stir well before adding the tinned tomatoes. Season with sea salt and black pepper and then add the half teaspoon of dried oregano and half tablespoon of chilli flakes. Simmer for 10 minutes stirring frequently.
Pour half the sauce into a shallow oven proof dish and carefully place the chicken on top. Cover with the remaining sauce and then tear over the mozzarella cheese and finally, sprinkle over the parmesan.
Cook for 15 minutes until the cheese has melted, then whack the heat up to its highest setting and cook for a further 5 minutes which will give the cheese a nice browning. Serve as it is or with some crusty bread. Enjoy!