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1
Work the semolina with the lukewarm water to form a firm, smooth dough. Knead on a board until the surface is silken and springs back when pressed. Wrap and rest at room temperature for at least 30 minutes. The dough is ready when it offers no resistance to folding and shows no dry streaks.
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2
Pinch a length of dough the width of a pencil and roll it against the board on a ferretto — a thin metal rod or knitting needle. Press firmly so the pasta wraps the rod; slide it free to leave a hollow, ridged tube. Lay finished maccheroni on a floured tray and do not allow them to touch. They are done when each piece holds its hollow and shows a faint groove along its length.
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3
Soak the stale bread in the whole milk for 10 minutes until fully saturated. Squeeze out excess milk lightly, retaining enough moisture to aid emulsification — the bread should feel sodden but not dripping.
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4
Place the garlic in the mortar with a pinch of salt and pound to a paste. Add the blanched walnuts in two or three additions, pounding each addition to a rough, pale cream before adding the next. Work in the milk-soaked bread and the marjoram leaves, pounding until the mass coheres. Drizzle in the olive oil in a slow stream, working it in with the pestle in a circular motion. This step decides the dish: pounding by hand preserves the walnut oils and prevents oxidative bitterness — a blender renders the sauce grey and astringent. The finished paste is rough-creamy, ivory in colour, and holds its shape when pressed with the pestle.
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5
Bring a large pot of well-salted water to a rolling boil. Cook the maccheroni al ferretto until tender but with a discernible resistance at the core. Reserve 120 ml of pasta cooking water before draining.
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6
Transfer the hot, drained maccheroni immediately to a warmed wide bowl. Add the walnut paste and two or three tablespoons of the reserved pasta water. Toss vigorously off the heat, working the paste into the pasta with the motion of the wrist, adding further pasta water by the spoonful if needed. The sauce must meet hot pasta: it is the residual heat of the maccheroni that melts the paste to a coating consistency. The dish is ready when each tube is evenly cloaked and no pooling appears at the base of the bowl.
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7
Divide immediately among four warmed plates. Finish with the grated pecorino and serve at once. The sauce oxidises and greys if allowed to stand — delay by even a few minutes is visible on the plate.