If you find yourself at the Fremont Street Fair desperately looking for something--anything--recognizable from the good old days before Quadrant began to run amuck, never fear: Costas Opa is celebrating its 20th anniversary this month, on the only corner of 34th and Fremont still intact. It continues to serve up consistent, affordable and enjoyable Greek delicacies with friendly service in a pleasant atmosphere, with a rare view of Waiting for the Interurban and America's busiest drawbridge.We started our meal with something set on fire at tableside: Saganaki. "Opa!" (Who said that?) This wonderful fried Greek cheese comes with a warning: If anyone in your party is extremely shy, don't order it, because everyone in the place will look at you when you get it. We had to try a couple of soups, as the Greeks make some incredible ones. We sampled the Fakes (lentil soup) and the Avgolemono (a lemony chicken, egg and rice soup). Both were as wonderful to taste as they are to pronounce.For the main course, we went with the Mediterranean Plate, an excellent way to sample a variety of flavors, and the Artichoke and Mushroom Linguine, a terrifically full-flavored vegetarian pasta dish that will please any palate. The Mediterranean Plate comes with roast lamb, which was flavorful, though a bit dry for my tastes; moussaka, a baked dish of eggplant, meat sauce and cheese that was a taste treat; dolmades, grape leaves wrapped around ground beef and lamb, wonderfully seasoned, and topped with a terrific avgolemono sauce; and spanakotyropeta, the classic fillo pastry filled with spinach, feta and egg--yum!Costas Opa, 3400 Fremont Ave. N, 633-4141. Open 7 days, 11am-10pm, Fri-Sat until 11pm.