In a large bowl, combine ground turkey, onion, bell pepper, egg whites, watercress, teriyaki sauce, and jerk seasoning until well blended.
With clean hands, form the turkey mixture into 6 equal patties.
Heat a non-stick griddle (or large non-stick pan) over medium-high heat and spray twice lightly with canola oil cooking spray. Place each patty on griddle, in batches if necessary, and cook for 3 minutes. Flip burgers and cook for another 3 minutes. Alternatively, cook burgers on an outdoor grill for 3 minutes on each side. Remove patties from griddle (or grill).
Spread 1/6 of the mayonnaise over the bottom half of each bun. Place burgers on buns and top each with 1 slice of tomato and other half of bun.