Part 1: Prepare the Meatball Mixture

1. Create the Panade: In a large mixing bowl, combine the Panko breadcrumbs and the whole milk. Stir them together and let the mixture sit for 5-10 minutes. This allows the breadcrumbs to fully absorb the milk, which is the secret to preventing dry meatballs. This hydrated mixture, called a panade, adds moisture and creates a tender texture.
2. Combine Ingredients: To the bowl with the panade, add the ground beef, ground pork, grated Parmesan cheese, beaten eggs, chopped fresh parsley, chopped fresh basil, minced garlic, salt, pepper, dried oregano, and optional red pepper flakes.
3. Mix Gently: Using your hands, gently mix all the ingredients until they are just combined. Be very careful not to overmix. Overworking the meat will develop the proteins (myosin) too much, resulting in tough, rubbery meatballs. Mix only until you no longer see streaks of individual ingredients.

Part 2: Form and Sear the Meatballs (The Flavor-Building Step)

1. Form the Meatballs: Lightly oil your hands to prevent sticking. Scoop the meat mixture and roll it into balls approximately 1.5 inches in diameter. You should get between 24 and 28 meatballs. Try to keep them uniform in size so they cook evenly. Place the formed meatballs on a baking sheet or large plate.
2. Heat the Pan: In a large skillet or Dutch oven, heat 2 tablespoons of olive oil over medium-high heat. The oil should be shimmering but not smoking.
3. Sear in Batches: Carefully place half of the meatballs in the hot skillet, ensuring you don’t overcrowd the pan. Searing in batches is crucial for getting a good brown crust. Cook for 2-3 minutes per side, until they are nicely browned all over. They do not need to be cooked through at this point; you are simply developing flavor and texture.
4. Transfer to Slow Cooker: Using a slotted spoon, transfer the browned meatballs directly into the basin of your slow cooker (a 6-quart or larger model is ideal). Repeat the searing process with the remaining meatballs and add them to the slow cooker as well. Do not wipe out the skillet.

Part 3: Prepare the Slow Cooker Sauce

1. Sauté Aromatics: Reduce the heat under your skillet to medium. Add the finely diced onion to the pan and cook for 4-5 minutes, stirring occasionally, until it has softened. Use your spoon to scrape up any browned bits (fond) from the bottom of the pan—this is pure flavor!
2. Bloom the Garlic and Paste: Add the minced garlic and tomato paste to the skillet. Cook for another minute until the garlic is fragrant and the tomato paste has darkened slightly in color. This “toasting” of the tomato paste deepens its flavor.
3. Deglaze the Pan: Pour in the red wine or beef broth. Bring it to a simmer and use a wooden spoon or spatula to scrape up any remaining browned bits from the bottom of the pan. Let the liquid reduce by about half, which should take 2-3 minutes. This step incorporates all the delicious flavor from searing the meat directly into your sauce.
4. Combine Sauce Ingredients: Pour the deglazed mixture from the skillet into the slow cooker over the meatballs. Add the two cans of crushed tomatoes, dried oregano, dried basil, sugar (if using), salt, and pepper. Gently stir everything together, trying not to break up the meatballs. Submerge the bay leaf in the sauce.

Part 4: Slow Cook to Perfection

1. Set and Forget: Cover the slow cooker and cook on LOW for 6-8 hours or on HIGH for 3-4 hours. The low and slow method is highly recommended, as it gives the flavors more time to meld and results in incredibly tender meatballs.
2. Final Touches: About 15 minutes before serving, remove the bay leaf. Taste the sauce and adjust the seasoning with more salt or pepper if needed. Stir in the handful of fresh basil leaves for a final burst of fresh, aromatic flavor.