Wash the liver, drain and gently remove all the films and large vessels. Cut the liver into 1 cm wide oblong pieces (the length of the pieces can be optional). Put the pieces of liver in a deep plate, salt and pepper. Add flour and stir, so that the pieces are floured. First, this coating will create a crust and protect the meat during cooking. Secondly, the flour will thicken the sauce and make it taste better. Melt butter in a frying pan and quickly fry the liver pieces under a high heat. A light crust should form on top. Quickly slice onions into semi-circles and add to the by-product. Cook the dish for a couple of minutes and add a tomato paste. Pour in some water so as to slightly cover the pieces of liver. Let the sauce boil and simmer for 3-4 minutes. Then add sour cream to the dish, stir, warm all together and remove from fire. Serve the liver stroganoff hot and freshly cooked.