I'm not sure what I find so compelling about the barbecue sandwich ($3.35) served at Demeris (2911 South Shepherd, 529-7326, and branches). It might be the sauce of tomatoes, Worcestershire, liquid smoke and lemons (added last to give heft and complexity). Or then again, it could be the accompanying potato salad ($1.05). It's a vivid, almost offensive yellow, that salad, made so by the presence of mustard. It also contains red peppers. And chopped pickles -- the reason it tastes so wonderfully briny. The people at Demeris are proud of their salad. And they're proud of their sauce as well. That's why you get so much of it. Which explains the waxed-paper wrapping: It prevents nasty spills. I have to say, though, it never seems to work for me. But I don't let that deter me. When I go to Demeris now, I always bring a change of clothes.