Preheat the oven to 190°C conventional oven/ 170°C fan-forced (374° Fahrenheit/Gas 5). Line a large baking tray with baking paper.
Roughly chop the mortadella and place into a food processor. Process the mortadella in the food processor until finely chopped.
To make the Polpette (meatball) mixture, place the breadcrumbs in a medium-sized bowl and pour the milk over to soak for a few minutes. Add the turkey mince (ground meat), mortadella, egg, Parmigiano Reggiano (or Grana Padano), and sage to the bowl with milk-soaked breadcrumbs. Season well with salt to taste, then use your hands to mix until well combined.
Roll 1 heaped tablespoon of the Polpette (meatball) mixture into a ball. Place onto a plate and repeat with the remaining mince mixture. Cover the plate of polpette with cling film (plastic wrap) and refrigerate for 30 minutes. This allows the polpette to firm up.
Remove the plate of polpette from the fridge. Pour enough canola oil to generously cover the base of the paper-lined baking tray. Place the polpette into the baking tray with oil. Place the 4 bay leaves amongst the polpette. Bake for 25 minutes.
Serve the Polpette di tacchino (baked turkey meatballs) immediately – buon appetito.