Farce à gratin is the richest and most intensely flavoured of the classical forcemeats, built on a base of seared liver — traditionally pork or chicken — that is cooked just past pink, then pounded and sieved into a smooth paste with butter, egg, and seasoning. The name 'gratin' refers not to a baked dish but to the brief, high-heat searing (gratiner) of the liver that develops Maillard compounds before the grinding stage. The liver must be impeccably fresh, trimmed of all sinew and membrane, and cut into uniform pieces for even cooking. It is seared in smoking-hot clarified butter with diced shallot, a sprig of thyme, and a bay leaf — 90 seconds per side, leaving the interior rosy. The seared liver is cooled slightly, then passed through a meat grinder on the finest plate twice, followed by a drum sieve (tamis) for absolute smoothness. Soft butter — equal to one-quarter the liver's weight — is worked in, followed by beaten egg as a binder and a generous seasoning of quatre-épices, salt, white pepper, and a splash of Cognac or Madeira. The finished farce should be smooth enough to pipe, rich enough to coat the palate, and intensely savoury with a clean liver flavour unmarred by bitterness from overcooked bile ducts. It is used as the primary forcemeat in country-style pâtés, as the inner layer of a pâté en croûte, and as the base for savoury mousses. A poorly trimmed liver or an over-seared one produces a bitter, grainy forcemeat that no amount of butter can rescue.
Sear liver to rosy, not well-done — Maillard on the outside, pink within. Double-grind then pass through tamis for absolute smoothness. Butter at one-quarter liver weight for richness. Quatre-épices, Cognac, salt, white pepper for classical seasoning. Trim all sinew and membrane before cooking.
Soak chicken livers in milk for 2 hours before cooking — the casein binds bitter compounds and produces a sweeter, cleaner flavour. For the smoothest result, process the sieved forcemeat in a food processor with the butter for 30 seconds after the tamis step. Always make a test quenelle: poach a spoonful in simmering water, taste, and adjust seasoning before committing the entire batch.
Overcooking the liver — bitterness from overcooked bile ducts is irreversible. Skipping the tamis — the forcemeat will be grainy no matter how fine the grind. Using cold, hard butter — it must be soft to incorporate smoothly. Under-seasoning — forcemeat served cold needs 20% more seasoning than hot preparations.
Escoffier, Le Guide Culinaire; Larousse Gastronomique; The Professional Charcutier (Marianski)