Fry the onion and garlic in the oil until softened. Add the oregano, tomatoes and sugar, a little splash of vinegar and some salt and pepper, then simmer for 20 mins until the sauce is really thick. Stir in the chicken and transfer to a baking dish.
Heat oven to 220C/200C fan/gas 7. Tear over the mozzarella in chunks, then scatter over with the breadcrumbs with a bit more ground pepper. Bake for 20 mins until the chicken is piping hot through and the top is golden and bubbling. eat with mash, jacket potatoes, rice or pasta.