Chullin 111 - Cooking a Liver
Blood absorbed in the meat of animals is permitted as long as it is not separated from the meat. Thus a it is permitted to eat a peace of raw meat, even though it contains blood. However, once the blood leaves the meat, it becomes prohibited. If it is later re-absorbed into the meat, that meat should not be eaten. Accordingly, meat may not be cooked until its blood has been exacted. There are two methods for exacting blood, by roasting the meat or by salting it. Both methods cause the blood to leave the meat, never to return.
The liver, however, is so saturated with blood that salting is ineffective in extracting all of it. Abaye said to Rav Safra, "When you go to the Land of Israel, ask them, what do they do with the liver." Rav Safra posed the question to Rabbi Zerika, and he related that he himself stewed a liver for Rabbi Asi. Abaye was not satisfied. He said, "I had no doubt that you can cook the liver by itself. Can you cook it with other meats?" Since this was never resolved, people don't cook liver together with other meats.
Art: Floris Gerritsz. van Schooten - A Still Life of Meat and Fish with a Cook