Don’t misunderstand. We are well aware of the fact that Palena, Harry’s Tap Room, Sonoma and many others in the D.C. area sling a pretty good hamburger. But when the Bosnians do it, the burger is elevated from mere junk-food/pub food status to an ethnic cuisine epiphany. Cosmopolitan—a Bosnian carryout in a shopping center on N. Kings Highway in Alexandria—serves a terrific pljeskavica, or “Bosnian Burger.” An enormous slab of mildly spiced beef is sandwiched…