-Two knives (one with teeth- one without)
Be careful ( I am NOT responsible if injured.)
Cut ,with a sawing motion, apple half way in the top with the knife with teeth so you don't accidentally cut further into apple.
Turn apple over and turn a quarter turn again cutting with the knife with teeth.
With the sharp knife pierce apple on one cut and drag knife two other cut. Repeat on other side.
Grab two parts and pull apart. Deepen cuts if needed.