Wash the liver and pat dry with kitchen paper. Cut the onions into thin rings. Put butter in a frying pan and heat over medium heat, when the butter starts to foam, add the onion rings and fry until translucent. Then remove and set aside.
Heat some more butter in the pan. Add the liver and fry for 4-5 minutes from the first side, then turn the liver and add the browned onion slices and fry again for 4-5 minutes.
Now turn up the heat and expose the liver to a good heat for a minute from the side that was seared first. The liver and onions should now be ready to serve. It can also be seasoned with pepper and salt afterwards.
However, I prefer the pure taste of liver and onion. It goes well with mashed potatoes.