Jus de rôti is the most honest sauce in the French kitchen — nothing more than the collected drippings of a properly roasted piece of meat, deglazed and strained. There is no thickener, no cream, no butter mount. The sauce is the animal itself, concentrated through Maillard reaction on the roasting pan's surface and emulsified only by the natural gelatin in the juices. The technique begins with the roast: the meat must be properly seared or started at high heat to develop fond on the pan bottom. Once the roast rests, the pan goes on the stovetop. Pour off excess fat — but not all; a tablespoon remains for flavour. Deglaze with a splash of stock, wine, or even water, scraping the fond with a wooden spoon. Reduce by half, strain through a fine chinois, and adjust seasoning. The result should be dark amber, intensely savoury, and thin enough to pool on the plate rather than coat a spoon. For beef, deglaze with red wine and beef stock. For chicken, white wine and chicken stock. For lamb, a splash of Madeira and lamb jus. The jus should taste unmistakably of the animal it came from — if it tastes like wine or stock, the fond was insufficient or the reduction too timid. A great jus de rôti is the mark of a cook who understands that the best sauce is already in the pan.
Fond is everything — high initial heat creates the flavour base. Deglaze with stock or wine appropriate to the protein. Reduce until intensely flavoured but still fluid. Strain through chinois — no solids in the final jus. No thickener — body comes from natural gelatin only.
If the fond is insufficient from a single roast, build it by roasting mirepoix trimmings in the same pan 20 minutes before the roast finishes. For a richer jus, reduce commercially made demi-glace by half and use that as the deglazing liquid instead of stock. Rest the meat on a wire rack over a tray — the collected resting juices are stirred into the jus at the last moment for maximum intensity.
Burning the fond — dark brown is correct, black is carbon and bitterness. Deglazing with too much liquid, resulting in a dilute, flavourless jus. Adding flour or cornstarch — this converts jus into gravy, a different preparation. Discarding all the fat before deglazing — a small amount carries flavour compounds.
Escoffier, Le Guide Culinaire; Robuchon, Le Grand Larousse Gastronomique