Tartare sauce in its proper French form is a handmade mayonnaise enriched with a precise brunoise of cornichons, nonpareil capers, fine herbs, and sometimes a whisper of anchovy. It bears no resemblance to the sweet, relish-laden commercial product. The base is a standard mayonnaise — raw egg yolk, Dijon mustard, a thread of neutral oil whisked to emulsion, finished with white wine vinegar or lemon juice. The garnish is folded in at the end, never blended: cornichons in 2mm brunoise, capers left whole if nonpareil or roughly chopped if larger, chives cut to 3mm batons, chervil and tarragon in fine chiffonade. The ratio of garnish to mayonnaise is generous — tartare should look loaded, not plain. The sauce is traditionally served with fried fish, but it also partners excellently with cold shellfish, crab cakes, and vegetable fritters. The acidity should be pronounced — tartare is meant to cut through the richness of fried foods. A good tartare tastes of pickles, herbs, and mustard before it tastes of oil. The mayonnaise base should be firm enough to hold its shape on a plate but not so stiff it clumps. Hold at 4°C and serve within 4 hours of preparation. The sauce should never be heated.
Handmade mayonnaise base — never commercial. Garnish folded in by hand, never blended. Cornichons in precise 2mm brunoise. Acidity forward — designed to cut fried-food richness. Generous garnish ratio — it should look loaded.
Add a few drops of the cornichon brine to the mayonnaise base before folding in garnish — it distributes pickle flavour throughout. If anchovy is included, pound it to a paste and incorporate into the yolk stage. For fish and chips service, make the tartare slightly looser than for plated fine dining — it should be spoonable, not quenellable.
Using sweet pickle relish instead of cornichons — wrong flavour profile entirely. Blending the garnish into the mayonnaise — destroys texture contrast. Under-seasoning the base — tartare needs more acid and salt than plain mayonnaise. Serving with non-fried dishes where its assertiveness overwhelms.
Escoffier, Le Guide Culinaire; Larousse Gastronomique