Belmont Lounge204 Belmont Avenue E, 322-5028The folks at B&O Espresso, Olive Way's revered coffee and dessert shop, are branching out. They've renamed the dining area--it's now B&O Bistro--and expanded the tapas-style menu. More important, they've opened up the classiest bar on Capitol Hill. Situated in the tiny alcove off B&O's main entrance, the BELMONT LOUNGE's lush purple curtains parted two weeks ago to reveal a vintage marble-topped curved bar, where a genteel bartender mixes fancy cocktails, keeps tiny bowls of pistachios filled, and spins jazz and blues records. It's the perfect place to slip away by yourself, to nurse brandy and a crème brôlée. AMY JENNIGES