In a small bowl whisk together mayonnaise ingredients.
Discard any bones in salmon and finely chop salmon by hand. In a bowl stir together salmon, mustard, gingerroot, soy sauce, and salt and pepper to taste and form into two 3-inch patties.
In a non-stick skillet heat oil over moderate heat until hot but not smoking and sauté patties 4 minutes on each side, or until just cooked through.
Transfer salmon burgers to buns and top with mayonnaise, onion, and tomato.
