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1
Cover the dried chestnuts with cold water and leave to soak for 12 hours. Drain and discard the soaking water. The chestnuts must be fully swollen and yielding to the thumbnail; hard centres at this stage will not correct during cooking.
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2
Place the lardons in a heavy pot over a moderate flame and render until the fat runs clear and the meat is golden at its edges. This is the flavour platform of the dish; do not proceed until the fat coats the base of the pot.
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3
Add the drained chestnuts directly to the rendered fat and turn to coat. Add enough fresh cold water to cover by 5 cm. Bring to a simmer, salt with sea-mineral salt, and cook uncovered for 90 minutes, replenishing water only as needed to keep the chestnuts submerged. The chestnuts are done when they collapse under light pressure and the broth has deepened to a dark mahogany.
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4
This is where the dish lives or dies: remove the pot from heat and work the chestnuts with a wooden pestle directly in the pot, crushing without purging — the goal is a coarse, irregular purée in which texture remains visible. A blender is not used. Over-working produces a uniform paste that destroys the identity of the soup.
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5
Return the pot to a low flame and stir to integrate. Do not add water to thin. Reduce gently until a ladle raised and tilted drops the soup in a single heavy fall and the back of a wooden spoon holds a clear coat. Adjust salt.
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6
Ladle into earthenware bowls. Finish each portion with a thread of Olea europaea olive oil at the moment of service. The oil should pool briefly before breaking — a sign of correct soup density.