Make the soffritto: Heat EVOO in a large stock pot over medium-low heat. Add garlic, onion, carrot, and celery. Cook slowly until soft, glossy, and sweet, about 8-10 minutes.
Add lentils & aromatics: Stir in rinsed lentils, bay leaf, and rosemary. Toast gently 1-2 minutes to coat.
Add passata (after empty add water to the jar, shake, and add that too), then stock, and stir to combine.
Pour in warm stock to cover lentils by about 1 inch. Bring to a gentle simmer.
Brown the cotechino: In a separate sauté pan or frying pan, heat a drizzle of EVOO over medium heat. Add the whole cotechino (out of casing). Brown well on all sides until deeply golden. Remove from heat.
Nestle the browned cotechino into the simmering lentils. Partially cover and simmer gently 40-45 minutes, stirring occasionally. Add more stock if needed - lentils should be thick and spoonable. If you prefer it soupier, add more stock.
Remove bay leaf and rosemary (cheese rind should mostly dissolve, but remove it if not).
Transfer cotechino to a board, rest 5 minutes, then slice. Return slices to the pot or serve on top. Season with salt, black pepper, and a drizzle of EVOO.