Heat the oil in a large frying pan. Add the onion, celery and carrots and cook for 8-10 minutes, stirring until softened. Add the garlic for the last 1-2 minutes, then leave to cool slightly.
Meanwhile, preheat the oven to 200ºC/gas mark 6. In a bowl, mix the turkey mince and veg with a wooden spoon and season well. Roll into 16 even-sized balls, each about the size of a walnut. Place the balls on a baking tray and cook in the oven for 25-30 minutes until golden brown.
Meanwhile, cook the tagliatelle in boiling water for 12 minutes until tender. Drain.
Pour the passata into a large pan and heat gently. Stir the meatballs into the sauce with the basil, then heat through and serve with the tagliatelle. Garnish with the remaining basil leaves.