Wash the apple and cut it into quarters. Peel off the skin and remove the seed core. Cut each quarter lengthwise in half.
Peel the onion and slice it. Heat the butter in a pan and add the onion. Fry for a few minutes, stirring frequently.
Add the apple and balsamic vinegar. Season with salt, pepper, cinnamon, and chilli. Fry for a few more minutes, stirring continuously.
Clean the liver (remove any membranes, veins, etc.). Then add the liver to the apple and fry over medium heat, covered, for about 10 minutes. Stir frequently and turn the liver over.
Transfer to a plate. If necessary, season again with salt.