You won't find much Salvadoran cuisine at Tres Banderas, an Ellisville restaurant that serves mostly Mexican fare. But you don't need much more than the pupusa, one of El Salvador's most famous dishes. A pupusa is a corn tortilla stuffed with cheese, meat or beans. Think of a tamale smushed into a quesadilla. Tres Banderas offers a choice of three fillings: cheese, chicken (with cheese) or carnitas. Whichever you choose, your pupusa will arrive topped with curtido (a cabbage slaw) and red salsa. The cheese pupusa is the most straightforward and perhaps the most appealing: The mild sweetness of the masa tortilla and the tangy cheese are nicely balanced by the tart slaw and the moderately spicy salsa. Two pupusas make a satisfying and — at just under $3 each — inexpensive lunch.