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1
Combine the shallot brunoise with roughly half the white wine vinegar in a small bowl. Leave to macerate for 10 minutes. The shallot is ready when its raw bite has softened and the vinegar has taken on a faint allium scent — sharp but no longer aggressive.
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2
In a mixing bowl, whisk together the Dijon mustard, the remaining vinegar, and the caper brine. Pour in the oil in a thin, steady stream, whisking continuously, until the base is loose and uniformly emulsified. Season with fine salt and white pepper. The texture should be fluid and spoonable, not thick — it coats a spoon but falls from it freely.
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3
This is where the dish lives or dies. Working with a sharp knife on a clean board, cut the chervil, tarragon, and parsley leaves to a fine chiffonade, and snip the chives, handling each as briefly as possible. No processor, no rocking blade — a single, decisive cut preserves cell structure. The herbs must retain their distinct colour: bright green, not darkened or bruised. Any bruising at this stage means the sauce will be bitter and dull within the hour.
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4
Stir the macerated shallot (with any residual vinegar) into the emulsified base, then fold in the whole capers, diced cornichons, and the cut herbs. Mix gently — a spoon, not a whisk — so the herbs remain distinct rather than dispersed into the oil.
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5
Cover and rest at cool room temperature for 30 minutes. Taste after resting: the flavours should be fully integrated — sharp acid, floral caper bitterness, tempered allium heat, and clean herb fragrance in balance. Adjust seasoning if required. Serve within 2 hours of assembly; beyond this the herbs darken and the flavour turns bitter.