4 Small, cored, peeled and cut into slices 1/ 2 inch ,1 centimeter thick
Heat half of the butter in a skillet and fry the bacon over medium heat until it is crisp and pale brown.
Remove the bacon from the pan and drain it on paper towels.
Season the liver slices with salt and pepper, and fry them in the bacon fat for a few minutes on each side five to seven minutes in all.
Transfer them to a warmed platter and sprinkle them with the Calvados, if using.
Cover the platter loosely with foil to keep the liver slices hot.
Add the remaining butter to the skillet and fry the onion rings and apple slices over medium heat until the onions are transparent and lightly browned about 10 minutes.
Spoon the onion and apple mixture over the liver and garnish it with the bacon.