Shortly after the steak frites at L'Entrecôte was set down in front of us, our friend la française summoned the waitress, who was uniformed in a black dress with a starched white apron to match the retro bistro décor.

"Is there not a lot of sauce tonight?" la française said in French, sounding concerned.

But of course there was, our waitress responded, and immediately ladled more of the secret sauce that makes L'Entrecôte so famous onto our plates.

After the waitress left, la française said, "That's how you ask for things in Paris: indirectly. If you ask them, 'Can I have more sauce on this?' they get all huffy."

Good advice for a restaurant that attracts more and more foreigners each season as word of its fabulous steak frites spreads. There is no real menu at L'Entrecôte; the waitress simply appears at your table and scribbles the number and doneness of your steak orders on the tablecloth.

Not many places could get away with serving just one thing - though there is this perfectly good mesclun salad with walnuts to start. But no one can get enough of L'Entrecôte's special butter sauce, first introduced to Paris in 1959 by Paul Gineste de Saurs. Since there are no reservations taken at L'Entrecôte, people line up around the corner and wait. Even on a Monday night, the place was packed, though the line moved quickly. We arrived at 8:30 pm and were seated in 30 or 40 minutes.

So what is the secret to this mystery sauce? Since the restaurant's inception the sauce recipe has been a closely guarded secret, handed down over the generations even as the family has branched out. Daughter Hélène Godillot took over the original 17th arrondissement restaurant, the Relais de Venice L'Entrecôte, her sister Marie-Paule Burrus started the rival Le Relais de l'Entrecôte, where we dined that night, and their brother Henri Gineste de Saurs opened a L'Entrecôte in Toulouse.

The intergenerational multiplication of L'Entrecôte doesn't seem to have affected the family in the same way as, say, the Manganaros of New York. Fortunately for diners everywhere, all the restaurants share the same recipe for a butter sauce from Geneva's Café de Paris in the 1940's.

As I tasted and retasted the sauce, trying to decode the delicious layers of flavor of herbs, cream, mustard, butter, and a perhaps a dash of white wine, I had no idea that we had arrived at L'Entrecôte amid a storm of controversy. Just weeks before, Jean-Claude Ribaut, restaurant critic for Le Monde, outed L'Entrecôte by publishing a recipe for the sauce. The secret, he said, is chicken livers. Not so, rebutted L'Entrecôte's Hélène Godillot in the London Independent. Make Le Monde's recipe and you will not have L'Entrecôte's sauce.

As for this copycat chef, I would concur with Jean-Claude Ribaut's guess at chicken livers, which certainly would explain the richness of the sauce and the odd greenish color. But I might also throw some sage and bay leaves, a grating of nutmeg, and a little white wine into the mix.

When dessert arrived - ultra-rich vanilla crème brulée scalloped to look like tarte tatin, scoops of freshly made sorbet - we were still puzzling over the secret to the secret sauce. But if any of us in the restaurant that night really knew the answer to this mystery, there would be no one waiting in line, and there has been a line outside of the various L'Entrecôtes in Paris day and night for 20 years now.

It seems there's only one way to get closer to the heart of the riddle: Go to Paris. Often.

i.e., Le Relais de l'Entrecôte
15, rue Marbeuf
Paris, France