On your stove top or on your outdoor grill, place a small sauce pot over moderate heat and cook onion in oil for 3 minutes. Add vinegar and reduce by half. Add syrup, tomato paste, Worcestershire, curry, smoke flavoring and pepper. Bring to a bubble and simmer.
Spray salmon with cooking spray and season with salt. Grill for 3 minutes, turn and baste liberally with glaze. Turn again, glaze opposite side. If you care for salmon pink at center, remove after 6 minutes. For opaque salmon, grill 5 minutes on each side.
Whisk citrus with garlic, mustard, tarragon and salt. Stream in oil to combine.
Spray romaine hearts with cooking spray and place on grill 2 minutes, charring the outside leaves. Remove and cut each heart in half lengthwise, using tongs to help hold on to the charred lettuce. Cut away core at stem. Cut each half heart in half again lengthwise. Place 2 quarters of charred romaine on each dinner plate. Drizzle with dressing, working back and fourth, strapping the lettuce with dressing. Set salmon on top of the salad. Garnish with chopped scallions and serve.