In a heatproof measuring cup, sprinkle gelatin all over surface of stock and let stand for 5 minutes (if not using stock and gelatin, proceed to Step 2). Microwave stock, stirring, once or twice, until gelatin completely dissolves, about 2 minutes. Pour stock into a wide heatproof bowl and refrigerate until fully set, about 30 minutes.

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 or grate 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/veal 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.

Line a rimmed baking sheet with aluminum foil. Divide the mixture and roll with wet hands.

Note: Mixture is quite sticky. Here is a trick for apportioning and rolling meatballs. Spread 1/2 mixture on parchment paper like a long log and then roll paper over the log to smooth it out and make it even. Now depending on what size meatballs you choose, use a knife to divide the log into 7 or 11 pieces. Now with wet hands roll these pieces into meatballs.

If serving later, store them at this point in the refrigerator

Preheat broiler and set oven rack in upper position.

Broil meatballs until browned on top, about 7 minutes (browning times can vary dramatically, depending on oven broiler strength).

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 5 minutes.

Serve meatballs, spooning sauce all over and grating more cheese on top.