KEENS STEAKHOUSE []

72 W. 36TH ST. (212) 947-3636

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MAYOR Mike, interviewed by the BBC last year, offered this advice for a long life: “If you want to stay healthy, stay out of the sun, don’t eat red meat and don’t smoke.”

Keens Steakhouse delivers on one-third of Dr. Bloomberg’s prescription: It definitely keeps you out of the sun. It’s hard to find a restaurant that feels more subterranean, thanks to dark wood, low ceilings and windows frosted to near-opacity.

But the 50,000 clay smoking pipes hung from the ceiling commemorate lung abuse down the generations. And Keens’ awesome mutton chop, the house special, ranks as Public Artery Killer No. 1. It’s the best reason I know to look forward to autumn’s shortened days, when the quickening chill calls for the richest red meat your system can take.

“At Neiman Marcus,” the lady at the next table said over a working lunch, “they have a saying – not that the customer is always right, but the customer is never wrong.” At Keens, mutton-eaters have not been wrong since 1885, when the place opened as a hangout for the theater district then concentrated around Herald Square.

In 1935, it sold its one-millionth order of mutton. In 1995, it changed its name from “chophouse” to “steakhouse,” but hedged its bet by keeping “chophouse” on the awning.

The wood paneling and leather banquettes and booths, although not quite as old as the room itself, cast you back in time. So do vintage theatrical and vaudeville posters; we can only wonder what we missed not being around for The Turkish Harem or Kelly & Leon’s Minstrels.

The act to catch today is the mutton chop ($35 at lunch or dinner), the four-star king of carnivorous joy. The meat of mature sheep (at least 2 years old) is a singular pleasure – as superior to lamb as a textured older Bordeaux is to a promising but unrealized younger bottle from the same chateaux.

Keens likes to serve it medium; let them. After 117 years, they’ve got the hang of it. The specimen, an inch and a half deep, arrives in a pool of its juice. If you worry about gaminess, don’t: Mutton is mustier than lamb, but it isn’t organ meat. Nor does it have the moldy quality dry-aged beef acquires if it’s hung a day too long.

Of course, it isn’t chicken, either. Expect deep, fat-sweetened flavor and surprisingly pliant texture. It doesn’t need mint jelly, but Keens’ is the best anywhere – freshly made, not green but amber and sharp enough for a mojito.

The rest of the menu is predictable – like shrimp cocktail ($14), hard to tell from the ice it’s served with, and nicely-marbled T-bone steaks ($37.50) bigger than some neighborhoods. Prime rib (in $23 or $35 cuts), like prime rib everywhere, recalls your last hotel wedding. Fresh limeade ($3.50) at lunch is a rare treat.

You can have steak anywhere, although the intense-flavored, Certified Angus Beef recently introduced at Bull & Bear in the Waldorf-Astoria throws down the gauntlet to other steakhouses.

But only Keens has mutton on the menu all the time. And the cold nights are coming.