Source: The Boston Cooking-School Cook Book (1896)
Melt 40g butter in a heavy-bottomed saucepan over medium heat. The butter should foam and smell toasty without burning — this is where your browning|browning starts. Add a few drops of onion juice and let it sizzle for 30 seconds, then dust in 45g flour. Stir constantly. You're building a roux that needs to turn nut-brown, not blonde. This takes 3–4 minutes of continuous work. The flour particles will toast and the mixture will darken noticeably. When it smells biscuited and looks the colour of light wood, you've hit the mark.
Pour in 240ml cold cream in a thin stream whilst whisking hard. Cold cream into a hot roux prevents lumps — the starch granules hydrate and thicken evenly. Keep whisking until the sauce is smooth and coats the back of a spoon. Set aside.
Whilst the roux cooks, trim 250g mushrooms. Peel the caps (this removes the fine skin and any grit trapped there), then halve them lengthwise and slice into half-moons. Heat 30g butter in a large frying pan over high heat until it stops foaming. The pan must be hot — you want the mushroom surfaces to caramelise, not stew. Work in batches if needed so the slices sit flat. Five minutes should give you golden edges and concentrated flavour. Season lightly.
Break the mushroom stems into 2–3cm pieces. Cover them with 180ml cold water in a small saucepan, bring to a bare simmer, and reduce slowly until only 40ml remains. This concentrates the umami and pulls out mushrooms|umami compounds that deepen the sauce. Strain through muslin. Dissolve 5g beef extract in this mushroom liquor — it dissolves best in warm liquid — then whisk the whole thing into the cream sauce. Taste and season with salt and a pinch of paprika for warmth and colour.
Fold in the sautéed mushroom caps just before service. They'll soften slightly in the warm sauce and stay intact, giving texture contrast rather than collapsing into it.
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