Dice the apples. That's a common way to do it for curry meatballs (curry sauce with meatballs served with rice and various toppings).
The apples make the sauce more filling and offsets some of the potential sourness of the onions/curry. It is all about the balance. I like to taste the sauce to perfection using salt, pepper, curry, sugar or syrup before adding the meatballs, but having apples in the sauce definitely makes it easier to begin with.