Heat the oil in a large frying pan. Add the onion, celery and carrots and cook for 8-10 mins, stirring, until the vegetables have softened. Remove from the heat and spread over a baking sheet to cool briefly. Meanwhile, preheat the oven to 200°C, fan 180°C, gas 6.
In a large bowl, mix the mince with the grated courgettes. Stir in the cooled vegetables and mix well. Divide the mixture into 3 equal batches and roll each into 24 balls, making 72 in total. Place the turkey meatballs on 2-3 large baking sheets and cook in the oven for 25-30 mins until golden brown.
Meanwhile, prepare 2 freezer bags and pour a third (about 465g) of the passata into each bag. Pour the remaining sauce into a medium-size pan and heat gently.
Place 150g of couscous in a bowl and pour over 175ml of boiling water. Cover and set aside for 10 mins, then fluff with a fork.
Stir 24 of the meatballs into the heated sauce with the basil and serve with the couscous.
Cool the remaining meatballs and divide between the 2 bags with the sauce. Seal and freeze for up to 3 months. To reheat, defrost thoroughly and heat gently in a saucepan until piping hot. Serve with 150g couscous and some fresh basil for each meal.