Heat 60ml of the oil in a large frying pan over a medium heat. Add the onions, garlic, and oregano, then cook for 5 to 6 minutes until the onions have softened and turned golden. Spoon half the mixture into a mixing bowl and set aside to cool. Add the tomato puree to the remaining onion mixture in the pan, along with the vinegar, chopped tomatoes, chipotle paste, brown sugar, and stock. Bring to a simmer over a low heat, season, then cook for 15 minutes.
Add the prosciutto or bacon, mince, fresh breadcrumbs and egg to the cooled onions in the mixing bowl. Season and combine well. With damp hands, form the mixture into walnut-sized balls and put on a tray
Heat the oven to 200C/180C fan/gas 6, then heat the remaining 40ml of oil in a clean frying pan over a medium to high heat. Fry the meatballs in batches, turning often, for 4 to 5 minutes until golden all over. Transfer them to the sauce and simmer for 20 minutes.
Mix the mozzarella, pecorino or parmesan, and panko breadcrumbs in a bowl.
To serve, prepare the polenta as per the packet instructions, then spread onto a greased baking tray, top with the meatballs and sauce, and sprinkle with the cheesy breadcrumb mixture. Bake in the oven for 5 to 6 minutes until the cheese has melted and the crumbs are crisp. Garnish with the chopped parsley.