When Chinatown Brasserie opened last year, its high production values and pointedly non-challenging menu misled many of the city’s self-appointed Asian food experts into dissing the place. But the city’s Chinese community was well aware that Joe Ng, the dim-sum chef there, was the city’s finest and one of the top in his field nationwide. (The process of making dim sum is a complicated one and demands its own chef and its own kitchen.) We dropped in on a recent Saturday morning to see how Joe did it and invited former Almond Flower chef Chris Cheung to help us with our eating.