In a medium saucepan, heat a drizzle of oil and sauté the onion with the bay leaf until soft. Add the garlic, wait for the aroma to exhale, add the peeled tomato and passata and cook for about 25 minutes on low heat.
In a bowl, mix the meat, breadcrumbs, parmesan and egg and mix well until a homogeneous mass. Form balls with the dough, make a hole, add the mozzarella and shape the polpetone. Season with salt and pepper.
Heat the oil in a very hot skillet and brown the polpetones for about 3 minutes on each side until golden brown. Put the sauce on top, basil leaves, tomatoes, and cover with the rest of the Parmesan. Place in the oven until well browned.