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1
Prick the fresh cotechino all over with a skewer at close intervals. Place it in a pot large enough to hold it flat and cover with cold unsalted water. Do not salt the liquid at any stage — the sausage carries its own heavy seasoning. Bring to the barest simmer over low heat.
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2
Maintain the gentlest possible simmer — no rolling boil — for 2 hours. The fat must render slowly into the cooking water; the casing must remain intact throughout. This is where the dish lives or dies: too vigorous a simmer and the casing bursts, the fat is lost to the water, and the sausage dries. The casing should be taut but unbroken, the sausage visibly plump.
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3
While the cotechino simmers, build the soffritto for the lentils: warm the olive oil in a wide saucepan over medium-low heat and add the finely chopped onion, carrot, and celery. Cook, stirring occasionally, until the vegetables are soft and translucent but have taken no colour.
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4
Add the dry Castelluccio lentils to the soffritto and cover with enough cold water to submerge them well. Bring to a simmer and cook until the lentils are tender but completely intact — they must hold their shape and must not become mushy. Castelluccio lentils are chosen precisely for this quality. Season with salt only at the end. The lentils are done when each one is tender to the bite yet retains its form.
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5
Lift the cotechino from its poaching liquid and allow it to rest briefly. Slice it thickly. Spread the lentils across a warmed serving dish or individual plates and lay the thick slices of cotechino over them. The rendered fat from the sausage will begin to run into the lentils immediately. The dish completes itself on the plate: the lentils absorb; the cotechino releases. Serve at once.