Grilled Salmon with watercress

3 pounds red potatoes small, thin-skinned 1 cup red onion thinly sliced 1 cup rice vinegar seasoned 1/2 pound watercress rinsed and crisped 1 each salmon[...]

3 pounds red potatoes small 1 cup red onion thinly chopped 1 cup rice vinegar 1/2 pound watercress rinsed and crisped 2 pounds salmon fillets 1 tablespoon soy[...]