Too often steak sandwiches seem an excuse to clear out inferior cuts of meat. Not at Quantum Kitchen & Cocktails (201 W. Carrillo St, 962-5999). This Flat Iron-admittedly a trendy cut, but in this case the trend is based on scrumptious fact-is a marbled marvel of flavor, gussied up just enough with some tarragon horseradish mustard (not too much of any of those easily overpowering flavors, either) and piled with tobacco onions, which just means they’re small, fried, and addictive. Do as one of S.B.’s best servers, Robert, suggests and order it as rare as possible.