Wash the liver, remove the fat and the films, and cut in half. Wash and peel onions and carrots. Cut carrots into strips, slice onions into semi-circles and crush garlic. Fry onion, garlic and carrots in vegetable oil for 10 minutes. Fry the liver until medium-done (about 10-15 minutes). Add onion, garlic and carrots to the liver, pour the sour cream over, salt and pepper. Stew under the lid for another 15 minutes. Serve with mashed potatoes or buckwheat porridge.