Put the liver in the milk to soak for a few hours.
In a saucepan, add 25g of the butter and cook the onions and apples. Add some salt and let everything cook over low heat until they are browned.
Drain the liver slices and flour them. Remove the onions and apples from the saucepan. Add the remaining 25g butter and brown the floured liver slices a little in the pan; season with salt. Stir in the onions and apples and cook until heated through. Turn off the heat and let the liver mixture rest for 10 minutes with the lid on.