Heat the oil in a frying pan over a medium heat. Add the meatballs and cook for 5 minutes, turning so they cook all over. Add the red onion and cook for a further 5 minutes, until the onion is soft and the meatballs are cooked through.
Toast the pittas and slice open on one side. Let everyone spoon in their own filling of tzatziki, salad, mint, tomatoes and meatballs, and serve with the lemon wedges to squeeze over the filling.