In the bowl of a stand mixer, combine bread with buttermilk, tossing to coat. Let stand, tossing occasionally, until bread is completely moist, about 10 minutes. Squeeze bread between your fingers or mash with a spoon to make sure there are no dry spots; if there are dry spots that refuse to moisten, add more buttermilk 1 tablespoon at a time until bread is moist throughout.
Add onion, garlic, Parmigiano-Reggiano, parsley, salt, pepper, pancetta, egg yolks, oregano, and fennel to bread/buttermilk mixture. Finely mince gelled stock, if using, and add.
Set mixer bowl in stand mixer and attach paddle. Starting at low speed and gradually increasing to medium-high speed, beat bread mixture until thoroughly blended, stopping to scrape down sides as necessary. Add 1/3 each of the beef and pork and beat at medium-high speed until thoroughly blended with bread mixture.
Remove bowl from stand mixer and add remaining beef and pork. Using a clean hand, gently mix meatball mixture, teasing apart ground meat with your fingers, just until ground beef and pork and thoroughly distributed throughout; avoid mixing any more than is necessary for even distribution.
Preheat broiler and set oven rack in upper position. Line a rimmed baking sheet with aluminum foil. Form meatball mixture into handball-sized balls and arrange on prepared baking sheet; you should be able to make about 10. Broil meatballs until browned on top, about 7 minutes (browning times can vary, depending on oven).
Heat tomato sauce in a medium pot until simmering and add meatballs. Simmer until meatballs are just cooked through and register about 145°F on an instant-read thermometer, about 10 minutes.
Serve meatballs, spooning sauce all over and grating more cheese on top.