Preheat a grill to medium-high heat. Coat the salmon with EVOO and season. Grill, covered, about 3 minutes. Flip the salmon over and grill until just cooked through, about 2 minutes more. Let the salmon cool completely, then flake with a fork.
In a medium bowl, gently stir the onion, mayonnaise, mustard and vinegar into the flaked salmon; season. Divide the watercress among the roll bottoms, top with mounds of the salmon salad and close with the roll tops.