Sauce Bercy for meat is the richer, more assertive counterpart to the fish-velouté-based Bercy, replacing the delicate fumet with demi-glace and adding bone marrow for an unctuousness that makes it one of the great bistro steak sauces. Named after the Bercy quarter of Paris — once home to the city's wine warehouses — the sauce begins with shallots sweated in butter without colour, deglazed with a full glass of dry white wine and reduced by two-thirds. Demi-glace is added and simmered for 10 minutes. The sauce is finished off heat with cold butter (monter au beurre) for sheen, a squeeze of lemon for brightness, and chopped flat-leaf parsley. The bone marrow — cut into 1cm dice, poached for 90 seconds in salted water — is folded in at the very end so it warms through without melting into the sauce. Each spoonful should deliver a piece of trembling, translucent marrow against the dark wine-scented demi-glace. The sauce is classically served with grilled entrecôte or thick-cut bavette, but it is equally at home with a pan-seared veal chop or grilled lamb rump. The white wine distinguishes it from the red-wine bordelaise, and the lighter acidity makes it more versatile with a range of proteins.
Shallots sweated without colour — they dissolve into the sauce. White wine reduced by two-thirds for concentration. Demi-glace for body, not espagnole — the sauce should be refined. Bone marrow poached separately and folded in at the end. Butter mount and lemon at finish.
If the bone marrow is too soft to dice after soaking, freeze it for 15 minutes to firm up before cutting. The marrow-soaking water (salted, with a splash of vinegar) draws out blood; change it twice over 2 hours for the cleanest result. For a modern presentation, place a single large round of poached marrow on top of the steak and spoon the sauce around — the marrow melts as the diner cuts into it.
Over-poaching the bone marrow until it melts — it should be warm and trembling, not dissolved. Using red wine — that makes it bordelaise, not Bercy. Under-reducing the wine — the sauce tastes thin and alcoholic. Browning the shallots — they should melt into the sauce, not add caramelised flavour.
Escoffier, Le Guide Culinaire; Larousse Gastronomique