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, 1 to 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, pancetta, Parmigiano-Reggiano, garlic, parsley, egg yolks, oregano, fennel, salt, and pepper 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, 1 minute. Add ⅓ 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 are thoroughly mixed in and no pockets of unincorporated meat remain; avoid mixing any more than is necessary for even distribution.

Preheat broiler and set oven rack in upper position. Line 2 rimmed baking sheets with aluminum foil. Form meatball mixture into tight handball-sized balls (about 2-inches wide and about 3- to 3 ¼-ounces each) and arrange on prepared baking sheet; you should be able to make about 20. Broil meatballs until browned on top, 7 to 10 minutes. (Browning times can vary dramatically, depending on oven broiler strength.) Repeat with remaining meatballs.

Heat tomato sauce in a medium pot until simmering and add half of the meatballs. Simmer until meatballs are just cooked through and register about 145°F (63°C) on an instant-read thermometer, about 10 to 12 minutes. Transfer to a serving bowl or platter. Repeat with remaining meatballs.

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