To prepare the salmon: Preheat the broiler. Rub both sides of the salmon with olive oil, salt and pepper. Broil the fish for 5 minutes on each side, or until almost cooked through. (The fish will continue to cook after you take the salmon out.) Remove and allow to rest for 15 minutes.
To prepare 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, pulse two or three times.
To assemble: Slice the buns in half and toast. Spread 1-2 tablespoons of the sauce on each cut side. Place some spinach on the bottom half, top with a piece of salmon. Place the top of the bun on the salmon.
