This is not Sarita's only creative pupusa. The long list of fillings includes spinach, mushrooms, broccoli, squash, carrots, potatoes and shrimp, in addition to the usual beans, chicharrón (pork) and loroco combined with cheese. They all come with cabbage relish (curtido). Bottles of hot sauce are on the counter.
You pay at the cash register and hold onto your receipt. When your order is ready, they'll call out the number -- in Spanish usually, but in English if nobody answers.
At $2.40, Sarita's pupusas are a little pricey. Most places charge under $2. And many have 99-cent specials. Perhaps you're paying for the privilege of watching your pupusas patted out by hand, possible only if you score one of the few seats at the counter.