Step 1

In a 6- to 8-quart slow cooker, blend the tomato paste and dry red wine until smooth. In a separate bowl, combine the panko breadcrumbs, parsley, Parmesan, grated onion, eggs, milk, garlic, red wine vinegar, and red pepper flakes. Season the mixture with ¾ teaspoon of salt and a generous pinch of black pepper.

Step 2

Incorporate the ground beef and sausage into the breadcrumb mixture. Using your hands, gently mix until evenly combined, taking care not to overmix to avoid dense meatballs. Shape the mixture into 7 evenly sized, lightly packed meatballs, approximately 3 inches in diameter. Arrange the meatballs in a single layer within the slow cooker. Pour the crushed tomatoes over the meatballs and place the basil sprigs on top. Cover and cook on low heat for 4 hours.

Step 3

Before serving, prepare the spaghetti according to the package instructions in salted boiling water, then drain. Increase the slow cooker to high heat. Using a slotted spoon, carefully transfer the meatballs to a bowl. Remove and discard the basil sprigs from the sauce. If needed, skim any excess oil from the sauce’s surface. Add the drained spaghetti to the sauce, tossing to coat evenly. Adjust the seasoning with additional salt and pepper, if required. Return the meatballs to the slow cooker, stirring gently to combine. Serve garnished with grated Parmesan and fresh basil leaves.