Cut the onion into rings. Heat 1 tablespoon butter in a pan and steam the onion rings in it for 5 minutes until translucent. Take out of the pan and set aside.
Peel the apples, remove the core and cut into 0.5 cm thick slices. Heat 2 tablespoons of butter in the pan, add the sugar and let it take on a little color. Add the apple rings and caramelize on both sides.
Pat the liver slices dry, turn them in the flour and fry them briefly on both sides in a second pan in the remaining hot butter over medium heat. Pour the calvados over the apple rings and add the onion. Season the liver with salt and pepper and serve with apples and onions.