BLUE LA LA

NO ketchup for you!

The Peter Luger of Paris has come to town, but those who like the red stuff with their steak frites are in for a shocker: The only condiment served is mustard.

The no-ketchup rule is far from the only quirk you’ll find at the new Le Relais de Venise L’Entrecote — the first American spinoff of a popular Parisian restaurant known as much for its long lines as its “secret” steak sauce.

The restaurant, located on the corner of 52nd Street and Lexington Avenue, serves only one dish: $24 steak frites.

Oh, yeah, and there are no reservations, no butter for your baguette and no steaks cooked medium rare.

It’s that kind of fanaticism that has garnered the restaurant legions of fans in Paris (where it’s known simply as “L’Entrecote”), as well as in London and Barcelona. And dyed-in-the-wool New Yorkers like Michael Lomonaco, the chef of Porter House New York, have been sighted within its somewhat harshly lit interior. But a few unsuspecting New Yorkers have been so taken aback by the brief menu that they’ve gotten up and walked out.

“Some love it and some hate it,” admits a staffer.

Still, bargain seekers should consider sticking around.

Rather than serving pricey prime dry-aged porterhouse steaks, Le Relais de Venise specializes in an affordable, oh-so-French preparation of choice sirloin served by waitresses in retro black-and-white uniforms.

Your only decision is whether to have it cooked well, medium, rare or blue — the barely cooked preparation rarely enjoyed stateside. Prefer medium rare? Your server will suggest medium.

Cooked on a grooved griddle, the steak arrives tender, sliced thin and drenched in a top-secret sauce with a recipe that is the subject of heated online debate. (A few years ago, the French newspaper Le Monde reported that chicken liver was the mystery ingredient — a claim the restaurant vociferously denies.)

What is known is that it contains butter, tarragon and, according to managing director Darin Nathan, “some citrus, herbs and spices.”

Whatever’s in it, you’ll find yourself craving it long after it’s gone.

The $24 price tag also includes sliced baguette from Balthazar, green salad with walnuts and mustard vinaigrette and a side of fries cut just an inch or two long to make them easier to fork into your mouth. (For vegetarians, $24 buys a salad, fries, a cheese plate and dessert.)

Still, portions seem shockingly small — that is, until your waitress arrives with more steak (it’s kept warm on a nearby burner), plus another helping of hot, freshly cooked fries.

Rounding out the meal is a selection of affordably priced wine (including a $19.95 bottle of house Bordeaux) and a surprisingly broad selection of desserts. Those include profiteroles in dark chocolate sauce with just a hint of spice ($6.50) and an assortment of cheese from Artisanal ($8.25) that comes with — sacre bleu! — butter.

Le Relais de Venise

590 Lexington Ave.

212-758-3989

relaisdevenise.com