"Free Dessert," says a little sign hanging in the window at Saigon Deli and Restaurant's University District location. But eat your meal first. You'll munch mostly on nostalgia for things you never knew you missed: the price of lunch in your junior high's cafeteria (delicious Vietnamese baguette sandwiches are $1.75), the winking Vietnamese grandmother you never had (co-owner Ngoc Le is as cute as they come), and pearly pink chopsticks (Barbie would love them). The free treat is a sort of Vietnamese panna cotta, really a silky, sweet, slightly tart homemade yogurt. Le makes it daily from sweetened condensed milk, water, and sugar, and serves it to every customer in a little plastic cup at the end of the meal, just because she loves us. If you're really nice, she'll pack up a second helping for you to take home.—Jess Thomson 4142 Brooklyn Ave. N.E., 634-2866, and other locations.