Cut each chicken breast in half lengthways so you have four fillets. Put the fillets on a board, cover with a sheet of baking parchment and bash with a rolling pin until they are the same thickness all over.
Put the flour in a shallow dish and season with a little salt. Tip the egg into another dish, then the breadcrumbs and parmesan into a third dish, stirring to combine. Working with one chicken fillet at a time, coat in the flour, then dip into the beaten egg, and finally coat in the cheesy breadcrumb mixture. Transfer the breadcrumbed fillets to a plate as you go. To freeze, stack the fillets in a freezerproof container between sheets of baking parchment. Will keep frozen for three months.
To make the sauce, heat the olive oil in a shallow pan and sizzle the garlic for 2 mins. Sprinkle in the sugar and add a splash of the vinegar, then tip in the tomatoes or passata. Add the oregano, season and bubble for 10-15 mins, stirring occasionally until thickened. To freeze, leave to cool completely, then transfer to a freezerproof container (portion it into more than one, if you like). Will keep frozen for up to three months. Defrost overnight in the fridge, then reheat in a pan until piping hot.
Heat the sunflower oil in a large frying pan over a medium heat and cook the breadcrumbed chicken fillets for 3 mins on each side (fresh or from frozen) until golden and crisp. Arrange on a baking tray. Spoon a little of the sauce over the middle of each fillet, and top each with a slice of mozzarella. Heat the grill to high. Cook the spaghetti following pack instructions, then drain and toss with the remaining sauce. Grill the fillets for 3-4 mins, or 1-2 mins more if they’re from the freezer, until the mozzarella is melted and bubbling, then serve.