Add first seven ingredients (bread crumbs through parsley) to a large bowl and stir to combine. Add sausage and ground beef, mixing well with your hands - but don't overmix, you don't want your meatballs to be tough.
Scoop mixture out onto prepared baking sheet in similarly sized ragged mounds with a 3-tablespoon scoop or heaped onto a standard tablespoon (these meatballs are extra large, about 3 bites each). Gently roll one mound into a ball between your palms. Repeat with remaining mounds, arranging the meatballs very close together, but not touching.
Bake 20 minutes until nearly cooked through.
Meanwhile, make the Garlicky Tomato Sauce: Add garlic and tomatoes with juice to a blender pitcher; blend until smooth. Add olive oil and blend again. Season to taste with salt and freshly ground black pepper. When meatballs are nearly done, heat sauce in a saucepan to simmer.
Remove meatballs from oven once they're cooked through, leave oven on.
Spread about ⅓ of tomato sauce into a large, oven-safe baking dish. Carefully transfer cooked meatballs to dish and pour remaining sauce over.
Sprinkle cheese evenly over meatballs and bake until cheese is melted, 5 to 8 minutes more.
Serve garnished with fresh basil leaves if desired.