There are lots of reasons to go to Petra Mediterranean Bistro, not least among them the stunning pictures of Jordan's ancient city, for which this elegant Belltown establishment is named. But, really, you need no other reason than the hummus?in particular the "mosabaha" hummus (there are three kinds). For one thing, this dish is so generously portioned that it can serve as a meal in itself. You get not only an overflowing plate of hummus but a pile of warm pita, in marked contrast to a lot of other joints that go skimpy on the bread, leaving you with either a spoon or your fingers to scoop up the rest. More important, this hummus is a decadent treat?for the most part as smooth as cream, but with whole chickpeas adding texture. Parsley lends a note of bright flavor. The pièce de résistance, though, is the fried pine nuts. As the Italians learned long ago, pine nuts make anything taste good. Frying them: even better. ?Nina Shapiro 2501 Fourth Ave., 728-5389