Though conventional wisdom says there are no good places to eat in the Sunset, insiders know better. Thanh Long's outstanding Eurasian/French colonial cooking draws a local crowd every night to its corner spot at Judah and 46th Avenue. The elegantly appointed eatery specializes in roast crab, as evidenced by the elegant silver crab cracker and pick at each place setting. Order the dish and you'll find yourself draped in a plastic bib, tied behind your neck whether you want it there or not. Our advice: Don't fight it -- you'll need it. The crab arrives open, topped with its own shell, in a shimmering pool of melted butter. Deep bowls at every seat fill rapidly with bits of shell; quick staffers spirit them away regularly. As your meal ends, servers bring warm washcloths for cleanup. If the place had showers, the repast would be perfect.