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1. In the bowl of a stand mixer, combine bread with buttermilk. Let stand, tossing occasionally, until bread is completely moist, about 10 minutes. Squeeze bread 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.
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2. Starting at low speed mix, gradually increasing to medium-high speed, 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.
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3. Remove bowl from stand mixer and add remaining beef and pork. By hand, gently mix meatball mixture, teasing apart ground meat with your fingers, just until ground beef and pork and thoroughly distributed. Avoid mixing any more than is necessary for even distribution.
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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 16. Broil meatballs until browned on top, about 8 minutes (browning times can vary dramatically). Transfer meatballs to a plate, cover with plastic wrap, and refrigerate until ready to use.
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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.
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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.
