Remove the film from the liver and remove the ducts, cut into small pieces and cook for 20 minutes until done. Dice the onion and fry in a frying pan with vegetable oil until soft.
Pass the liver through a fine mesh of a meat grinder, do the same with the fried onions and boiled carrots. Add salt, pepper, mix until smooth.
Spread parchment on the table and lay out the liver so that you get a rectangular layer 0.5-0.7 mm thick. Leave the edges of the parchment free. Put butter on top of the liver, leaving 2 centimeters from the edge.
Roll up the roll, using parchment to help you. Fold the edges of the parchment at the ends. Put the roll in the refrigerator to cool for at least 4 hours.