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1
Bring a pan of water to a full boil. Add the walnut kernels and submerge them for 2 minutes exactly. Drain, then rub the warm kernels in a clean cloth: the thin, papery inner skin will slip away. Work while the walnuts are still warm — the skin is easier to remove. The walnuts are ready when they are pale, ivory-cream and free of any grey or brown membrane.
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2
Soak the crustless bread in milk, then squeeze it firmly so no excess liquid remains. The bread should hold together as a moist, compact mass — not wet, not dry.
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3
Place the blanched walnuts in a mortar with the crushed garlic and a pinch of salt. Work to a coarse paste. Add the squeezed bread and marjoram and continue to pound. The paste is sufficiently worked when no large walnut pieces remain and the mass holds together cohesively. For a smoother result, transfer to a blender and pulse briefly — do not over-process.
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4
Work in the prescinseua or ricotta and the grated Parmigiano. Stir to combine. Stream in the olive oil gradually, working it in as you would a mayonnaise, until the sauce is thick, creamy, and pale. Taste for salt. The finished sauce should coat a spoon and smell of marjoram and sweet walnut, with no trace of bitterness.
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5
If dressing fresh pasta, thin the sauce with a ladleful of hot pasta cooking water and toss immediately. The sauce should cling to each strand or parcel without pooling. Use at once — walnut oils oxidise within hours and the sauce will darken.