Sauce vénitienne is a delicate green-tinted fish sauce built on a white wine reduction perfumed with tarragon and chervil, finished with a herb butter that gives it its characteristic pale jade colour. The sauce begins as most fish sauces do: shallots sweated in butter, deglazed with white wine and tarragon vinegar, reduced by two-thirds. Fish velouté is added and simmered for 10 minutes. The defining element is the beurre vert — green herb butter made by blanching tarragon and chervil leaves in boiling water for 10 seconds, shocking in ice water, then pounding in a mortar with softened butter until the mixture is smooth and vividly green. This green butter is whisked into the strained sauce off heat, turning it from ivory to a subtle sage-green that gives the sauce its name and its visual identity. The flavour should be delicate: tarragon's anise note should thread through the sauce without dominating, chervil should contribute its mild, almost parsley-like freshness, and the fish velouté base should carry the seafood character. Sauce vénitienne is the classical accompaniment for poached fish — turbot, sole, salmon — and demonstrates the saucier's ability to build complexity from simplicity. The green must be natural and subtle; if the sauce looks like pesto, the herb butter was too generous.
White wine and tarragon vinegar reduction as flavour base. Fish velouté body. Beurre vert (green herb butter) for colour and herbal fragrance. Blanch herbs before pounding — preserves colour, removes raw bitterness. Subtle jade colour — not vivid green.
Spin the blanched herbs in a salad spinner to remove all water before pounding — excess moisture dilutes the butter and makes the colour cloudy rather than vivid. A teaspoon of spinach juice (raw spinach processed and strained) added to the herb butter intensifies the green without adding detectable flavour. Strain the finished sauce through a fine chinois after the butter mount for absolute smoothness.
Using raw (unblanched) herbs in the butter — the colour will be dull and the flavour harsh. Adding too much herb butter — the sauce looks like pesto, not vénitienne. Using dried herbs — no colour, no fragrance, no point. Boiling the sauce after adding the herb butter — the colour turns brown and the herb flavour turns bitter.
Escoffier, Le Guide Culinaire; Larousse Gastronomique