Heat the grill to high.
Brush the salmon with the oil and season with salt and pepper. Place the salmon on the grill, skin side down, and grill until golden brown and slightly charred, about 3 minutes.
While the salmon is cooking, place the watercress and onion in a medium bowl, add the vinegar and oil and salt and pepper and toss to combine. Transfer the salad to a platter, top with the salmon fillet and drizzle each fillet with the mustard sauce.