Preheat the slow cooker if necessary; see the manufacturer's instructions. Make the meatballs. Put all the ingredients except for the oil in a bowl and mix with a wooden spoon. Wet your hands and shape the mixture into 20 balls.
Heat the oil in a large frying pan, add the meatballs and cook over a high heat, turning until browned on all sides. Lift them out of pan with a slotted spoon and transfer to a plate.
Make the sauce. Add the onion to the pan and fry, stirring, for 5 minutes or until lightly browned. Add the garlic, tomatoes, sugar, stock and salt and pepper and bring to the boil, stirring.
Transfer the meatballs to the slow cooker pot, pour over the hot sauce, cover and cook on low for 6–8 hours. Garnish with basil leaves and serve with tagliatelle tossed with chopped basil and melted butter.