At Chez René in Paris, they served sauce Bercy on the side, and it was so heavy with shallots that, at first, I thought it was just shallots, with some kind of warm vinaigrette added. But it isn’t: It’s a proper, cooked sauce of shallots with white wine and fond de veau. Just to drive you crazy, there’s a completely different sauce, also called sauce Bercy, made with white wine shallots and a fish veloute, and used, obviously, on fish.
My experience of entrecôte and sauce Bercy left me with the permanent conviction that a steak is not a steak at all if it is not sauced with shallots. In fact, my one improvement on the thing is to save some caramelized shallots from the pan to sprinkle on top of the steak in addition to the ones that sit in the sauce.
Heavily salt the entrecôte. Cook, preferably in a carbon steel pan, for four minutes on each side. (If it's sufficiently thick, and the window is open, do another three on each side.)
Meanwhile, cook the chopped shallots slowly, in a mix of butter and olive oil, in—this is key—a separate pan. (The mistake is to do them in the hyper hot pan that the steak has cooked in.)
When the onions are sufficiently caramelized (they should be browned), reserve about one-third of the shallots, and add the half cup of white wine to the rest. Boil off until almost dry. Then, add the cup of veal stock and reduce until the right (!) consistency—thick enough to coat a spoon without being glutinous. You can add just a touch of potato starch if it's too watery.
Serve the steak with the sauce, and the reserved shallots (delicious in their dry, browned state) on top. Pepper if you like. And some even add a squeeze of lemon to the sauce.