Place a grill wok on the barbeque. Preheat the grill to medium-high. Oil the grate beside the grill wok for the salmon.
Coat the vegetables with oil. Season with salt and pepper. Grill the potatoes in the wok for 10 minutes. Add the asparagus and green onions, and continue cooking for 5 to 8 minutes or until the vegetables are tender, stirring occasionally.
Meanwhile, grill the salmon skin-side down for 6 minutes, flipping halfway through the cooking time. It will be medium-rare.
Transfer the vegetables to plates, and top with the salmon, radishes, and watercress. Drizzle with the mustard mayonnaise.