Soak the lentils in a bowl of water for 3-5 hours.All dry lentils are different, so see if there are any instructions on the back of the package that indicated how many hours to soak (or if they need to be soaked at all)
Place a pot of cold water on the stove. Pierce the cotechino with a toothpick all over and wrap the cotechino in aluminum foil.
Place the cotechino in the cold water and turn the heat on to medium heat. Once the water starts boiling, leave it to boil for about 1.5 hours.
In a pot large enough to fit the lentils when they expand, add the olive oil over medium heat.
Once olive oil is heated, add the onion, carrots and celery. Let this cook through until tender, about 5 minutes.
Drain the lentils from the soaking water and add them to the pot with celery, carrot and onions. Season with salt and pepper.
Cook for a few minutes and add the tomato passata, bay leaves and rosemary. Let all the ingredients meld together for a few minutes.
Add a few ladles of warm vegetable stock. Stir, and once the stock has absorbed, add more.
Repeat this process of adding the stock until the lentils are cooked.Depending on your lentils, this can take anywhere from 15 mins to 40 mins.
Adjust for salt and pepper.
Remove the cotechino from the aluminum foil, let it cool slightly and remove the rind. Slice the cotechino.
Place the cooked lentils on a serving dish and place the sliced cotechino on top. Serve and enjoy!