Sixth Avenue in the mid-40s is a hub of Manhattan street meat, with competing chicken-lamb-and-rice carts occupying practically every corner. But only one cart is operated by a chef’s-jacketed, floppy-toqued veteran of the Russian Tea Room. Mohammed Rahman runs his gleaming silver box like a mini-restaurant, expediting orders to his busy crew and chatting up customers. The Bangladeshi immigrant’s claim to fame is his marinated lamb, a succulent triumph of cumin, coriander, yogurt, and green papaya, rolled up with yogurt-y white sauce in a puffy pita or served over basmati rice. His falafel is idiosyncratic and more Greek than Middle Eastern, what with tsatsiki subbing for tahini and the kind of pita you’re likelier to find at Greek gyro stands than Israeli falafel joints. Regardless, it’s a tasty, rich sandwich, and, along with his distinctive jalapeño hot sauce, part of the winning repertoire that’s enabled the sidewalk chef to branch out with two more midtown carts.