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1
Place the cleaned abomasum in a large pot with the onion, celery, carrot, parsley, and black peppercorns. Cover with cold water. Bring to a steady simmer over medium heat. The surface should show a consistent, gentle agitation — not a rolling boil, not a still liquid. Skim any grey foam that rises in the first ten minutes.
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2
Maintain the consistent simmer for a minimum of 90 minutes. This is where the dish lives or dies: the abomasum must reach complete gelatinous tenderness. Press the thickest part with a spoon — it must yield without resistance. Any chewiness means the boiling must continue. There is no shortcut and no recovery from under-cooking at the service stage.
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3
While the abomasum cooks, prepare the salsa verde. Pound or process the parsley, anchovies, capers, garlic, and vinegar-soaked bread to a coarse paste. The sauce should be bright green, cohesive, and sharp on the tongue — the acidity of the soaked bread and the salt of the anchovies must be clearly present.
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4
Remove the cooked abomasum from the broth and slice it thin. Keep the broth hot over low heat. Do not strain it yet — it is the service medium. The sliced abomasum should be glossy, deeply coloured, and collapsing slightly under its own weight.
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5
For bagnato doppio service: split each roll without fully separating the halves. Dip the interior crumb face-down into the hot broth for several seconds until thoroughly moistened — the bread should be saturated but not disintegrating. Then pass the sliced abomasum briefly through the broth to moisten it equally. The bread must be wet to the touch, yielding, and fragrant with the broth aromatics.
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6
Fill each roll with the broth-moistened abomasum. Apply salsa verde generously across the top of the filling. Close the roll and serve immediately. The sandwich should be heavy with broth, the salsa verde sharp against the gelatinous meat, the bread barely holding its structure.