Mix together half the onions, the raisins, minced beef, tomato purée and curry powder in a bowl. Using your hands, knead to combine the mixture evenly. Form the mixture into 24 meatballs.
Heat 1 tablespoon of the oil in a saucepan, add the meatballs, in small batches, and cook until browned all over. Tip out the excess fat and put all the meatballs in the pan. Add the cinnamon, tomatoes and lemon juice, cover and simmer gently for 25 minutes until the meatballs are cooked.
Meanwhile, heat the remaining oil in a large frying pan, add the celery and courgettes and cook until soft and starting to brown. Add the peas and cook for a further 5 minutes until the peas are tender.
Just before serving, stir the courgette mixture into the meatball mixture.