Step 1

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.

Step 2

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.

Step 3

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.

Step 4

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.

Step 5

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 distributed throughout; avoid mixing any more than is necessary for even distribution.

Step 6

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

Step 7

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.

Step 8

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