In a large bowl, gently combine the beef, sausage, panko, Parmesan, parsley, milk, egg, garlic, lemon zest, 1 teaspoon of salt, and ½ teaspoon black pepper.
In a 5- to 7-quart slow cooker, drizzle some olive oil in the bottom of the insert. Using your hands or a 2-inch ice-cream scoop, portion out the meatballs and arrange in the bottom of the slow cooker. You should have about 16 meatballs.
Using your hands, crush the tomatoes into the slow cooker and add their juices. Add the onion, butter, tomato paste, basil sprigs, dried oregano, crushed red pepper, and a generous pinch of salt. Do not stir.
Cover the slow cooker and cook on HIGH for 4 hours or on LOW for 6 to 8 hours, until the meatballs are cooked through and the sauce has thickened. (If desired, you can stir the sauce halfway through cooking, but it is not necessary.) Discard the onion and basil sprigs. Gently stir the sauce and season generously with salt.
Serve the meatballs over cooked pasta, polenta, or piled on a toasted hoagie bun. Garnish with freshly grated Parmesan cheese and finely chopped parsley.