Slow-Cooker Meatballs

Preheat oven to 425 degrees F. Combine panko, Parmesan, parsley, milk, egg, and 3 garlic cloves in a bowl; let stand 10 minutes. Add beef and sausage; gently combine using your hands. Shape into 36 (1- to 1 1/2-inch) balls. Line a rimmed baking sheet with foil and lightly grease with canola oil. Place meatballs on baking sheet and bake until browned, 15 minutes. Drain well. Combine onion, olive oil, Italian seasoning, red pepper, and remaining 4 garlic cloves in a microwave-safe bowl. Microwave, stirring once, until onion is tender, 3 to 4 minutes. Combine onion mixture, crushed tomatoes, diced tomatoes, tomato sauce, and sugar in the bowl of a 6-quart slow cooker. Gently stir meatballs into sauce. Cover and cook on low, until the flavors are combined, 8 to 9 hours. Stir in basil.