Step 1

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. Add half of onion, 8 cloves of minced garlic, Parmigiano-Reggiano, parsley, salt, pepper, egg, and half of oregano to bread/buttermilk mixture.

Step 2

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 3

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.

Step 4

Preheat broiler and set oven rack in upper position. Line a rimmed baking sheet with aluminum foil. Set aside 6 ounces of meatball mixture. Form remaining meatball mixture into golfball-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). Transfer meatballs to a plate, cover with plastic wrap, and refrigerate until ready to use.

Step 5

Heat oil in a large Dutch oven over medium-high heat until shimmering. Add remaining meat mixture and cook, stirring, until no longer pink, about 3 minutes. Add remaining onion and garlic and cook, stirring, until softened, about 5 minutes. Add remaining oregano and pepper flakes and cook, stirring, for 30 seconds. Add tomatoes and bring to a simmer. Season with salt.

Step 6

Transfer tomato sauce and basil sprig to slow cooker. Cook on low heat for 6 to 10 hours. Gently fold meatballs into mixture for last 30 minutes of cooking. Serve with pasta and Parmesan cheese.