Wash the lentils and leave them in the water for 12 hours.
Pierce the Cotechino with a toothpick and place it in a pot filled with cold water; boil it at low heat for 2 hours from when the water starts boiling (with a pressure cooker, 40 minutes from the whistle will be enough).
In the meantime, chop up an onion, put in a pan with a bit of olive oil and a laurel leaf. Let it brown. Add the lentils and cover them with the broth.
Cook for about 30 minutes until the broth will evaporate and the lentils will be tender. Remove the Cotechino and let it cool.
Cut it and put it in the blender container with 3 tablespoons of milk: blend it until it becomes a cream. In a small glass put 2 tablespoons of lentils and, with the help of a sac à poche, cover with the Cotechino mousse.