Combine minces, onion, garlic, parsley, breadcrumbs and egg in a bowl and mix together well using your hands. Roll into large meatballs and place into the slow cooker.
Add tomatoes, tomato paste and chopped basil and season with salt and pepper. Cover and cook on low setting for 6 hours.
When ready to serve, preheat grill to high. Transfer meatballs and sauce to a large heatproof dish. Scatter with mozzarella and parmesan. Grill for 6 minutes or until golden. Scatter with extra basil and serve.
No slow cooker? Brown meatballs in a frying pan over medium heat, turning often, until well browned. Add tomatoes, paste and basil and bring to the boil. Transfer to an ovenproof dish and bake in a preheated oven (180°c) for 25 minutes or until meatballs are cooked through. Top with cheeses and bake for a further 10 minutes until melted.