Sauce nantaise is beurre blanc's lesser-known precursor or sibling — a Loire Valley white wine and shallot reduction mounted with butter, traditionally served with the freshwater pike (brochet) that defines the river cuisine of Nantes and Anjou. While beurre blanc has achieved global recognition, sauce nantaise maintains a more specific regional identity and a slightly different emphasis: where beurre blanc often uses vinegar as a co-acid, nantaise relies solely on the acidity of Muscadet wine, giving it a rounder, less sharp character. The shallots are sweated in butter, deglazed with Muscadet sur lie (the lees contact gives a yeasty richness that elevates the reduction), and reduced to 2-3 tablespoons of concentrated wine essence. Cold butter in 1cm cubes is whisked in off heat, one cube at a time, building the emulsion as for beurre blanc. The finished sauce is strained and seasoned with fine sea salt from Guérande (the local salt — terroir extends to seasoning in Loire cuisine). A few whole chive batons laid across the sauce at service are the traditional garnish. The sauce should be paler than beurre blanc (no vinegar means less aggressive acid and paler colour), taste of Muscadet and shallot with a butter richness that does not overwhelm, and pair exclusively with fish — it has no affinity with meat.
Muscadet sur lie as the sole acid — no vinegar. Reduce to 2-3 tablespoons of concentrated wine essence. Cold butter whisked in off heat, one cube at a time. Strain for smooth finish. Guérande sea salt for authentic Loire seasoning. Fish-only — no meat pairings.
If Muscadet sur lie is unavailable, any dry, minerally, unoaked white wine will work — Chablis or Picpoul de Pinet approach the required profile. The lees character of Muscadet sur lie can be approximated by adding a pinch of nutritional yeast to the reduction — unconventional but effective. For pike (the traditional fish), poach the fish in a court-bouillon made with the same Muscadet used in the sauce — this creates a flavour through-line from poaching liquid to sauce.
Adding vinegar — that makes it beurre blanc, not nantaise. Using a non-Loire wine — the Muscadet character is the sauce's identity. Overheating during the butter mount — the emulsion breaks. Using salted butter — Loire tradition uses unsalted butter with separate Guérande salt.
Larousse Gastronomique; Clémence Lefeuvre (attributed); La Cuisine Nantaise