Heat oil in a Dutch oven over medium-high heat until shimmering. Add onions, tomato paste, garlic, oregano, pepper flakes, and salt and cook until softened and lightly browned, about 8 to 10 minutes. Transfer half of the onion mixture to a large bowl and set aside.

Add wine to the remaining onion mixture in pot and cook until slightly thickened, about 2 minutes. Stir in tomatoes, then transfer to a slow cooker.

Add sausage, mozzarella, parmesan, eggs, garlic and salt to bowl with reserved onion mixture. Mash with a potato masher until smooth. Add beef and cream to bowl and knead with hands until well combined. Lightly shape mixture into 2-inch round meatballs (about 12 total). Microwave meatballs on large plate until fat renders and meatballs are firm, 4 to 7 minutes. Nestle meatballs into slow cooker, discarding rendered fat. Cover and cook until meatballs are tender and sauce is slightly thickened, 4 to 5 hours on low. Let meatballs and sauce settle for 5 minutes, then skim fat from surface and stir in Parmesan and basil. Season with salt to taste. Serve.