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1
Melt the butter in a heavy saucepan over gentle heat. Add the flour all at once and stir constantly with a wooden spoon or whisk for 2–3 minutes. The roux is ready when the raw flour smell has fully dissipated and the mixture remains completely white — no colour of any kind must develop.
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2
Add a small splash of warm fumet to the roux and whisk vigorously until a smooth paste forms. Add a larger addition and whisk again until perfectly smooth. Pour the remainder in a steady stream, whisking throughout. The sauce at this point should be fluid, lump-free, and uniform — no white specks visible against the pan.
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3
Bring the velouté to a gentle simmer and maintain it there for 20–30 minutes, skimming the surface regularly as starch impurities rise. This is where the dish lives or dies: the cooking must not extend beyond 30 minutes — the delicate fumet flavour degrades with extended heat — yet it must not fall short, or the flour taste will persist on the palate.
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4
Pass the finished velouté through a chinois, pressing gently. The sauce should be ivory in colour and fall from a spoon in a silky, unbroken stream — no opacity, no graininess.