Put the liver in a saucepan with water and bring to a boil. Let it boil briefly, then take out the liver, drain it and cut it roughly.
Heat the butter in a saucepan and sweat the chopped onion and garlic in it for a few minutes. Then add the herbs and pieces of liver and pour about 100 ml of water over them. Cover and simmer over a moderate heat for about 15 minutes. The liquid should have largely evaporated.
Take out the herbs, add the egg yolks, puree the whole thing with the hand blender. Season to taste with salt and pepper.