In the bowl of a stand mixer, combine bread with buttermilk and veal stock, 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.
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, pork and veal 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 gloved 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.
Line a half-sheet pan with aluminum foil.
Form meatball mixture into handball-sized balls and arrange on prepared baking sheet; make about 10.
Under the salamander or broiler, broil meatballs until browned on top.
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.