For the sauce, place the mayonnaise, sour cream, vinegar, basil, dill, scallions, salt, and pepper in the bowl of a food processor fitted with a steel blade. Process until combined. Add the capers and pulse 2 or 3 times.
03To assemble the sandwiches, slice the rolls in 1/2 crosswise. Spread a tablespoon of sauce on each cut side. On the bottom 1/2, place some mesclun salad and then a piece of salmon. Place the top of the roll on the salmon and serve immediately.