First came Rocket, whose menu is little more than coffee, eggs benedict and rye bread sandwiches. So it was natural that the same team would open a proper restaurant just next door, Lady Brett. But the mini-hipster enclave actually has a third feather to its cap, U.N.C.L.E., located above Lady Brett, and accessible through a very speakeasy-worthy series of stairs and backdoors.

U.N.C.L.E, a wonderfully moody lounge with a stellar cocktail list, also serves some tasty steak and mussels. Fittingly, the space is all chocolate tones, leather upholstery, circular booths and warm lighting. An elegant hostess presides over it all, while a single barman in a classic vest stands behind the bar. In keeping with the dramatic atmosphere, diners are presented with a black envelope after taking their seats.

Inside, a tick-box menu offers two steaks, and mussels in two flavors. And that’s it, although you can order from Lady Brett downstairs. Why would you, though? The ku sirloin steak (B850) is tender, comes rare when ordered rare, and packs a ton of flavor. (Ku beef is locally raised by the Kasetsart University cooperative.) In fact we prefer it to the much more expensive US Prime “St. Helen” tenderloin (B2,500), which is more tender but less flavorful.

The mussels (B790) are just as satisfying, with the spicy tomato sauce particularly well-suited to battered Thai palates. They can also be ordered in a classic mariniere white wine sauce. Too bad the thin, McDonald’s-esque fries are so lackluster. When your menu is so limited, everything should be perfect: chunky, wedge potatoes would have been much more appropriate to U.N.C.L.E’s nostalgic Sinatra and martinis vibe.

Cocktails, too, have a rich, decadent quality, such as the Rowdy Tattooed Men (B395), which combines rum, cacao essence and peanut butter in what’s almost a Christmas-season dessert. The more classic Tea Trader (B395) packs quite the punch, too—a take on the classic Sazerac, it smoothly blends Hennessy cognac with chamomile tea, calvados, absinthe and Poire Williams.

We’re slightly awed by the fact that someone would sneak an old-school steakhouse into a bar, rather than go for a small menu of tapas and shared plates. The result is a lavish experience worthy of a Mad Men episode. When you’ve got the cash and the time to splurge on the more decadent things in life, you can’t really go wrong with this charming speakeasy.