2 c. mesquite wood chips -- soaked in water (optional)
salt and pepper to taste
In a 6-quart pan, bring about 2 quarts water to a boil over high heat; add the potatoes. Cover and simmer over low heat 15-20 minutes. Drain and chill.
Soak the onions about 15 minutes in cold water. Drain the onions and mix them with rice vinegar. Cut the potatoes in quarters and add them to the onions.
Trim the watercress sprigs from the stems, then finely chop enough of the stems to make 1/2 cup (discard the extras or save them for other uses). Mix the chopped stems on a large oval platter with the potato salad alongside. Cover the mixture and keep cool.
Rinse the salmon and pat dry. Place the salmon, skin side down, on a piece of heavy foil. Cut the foil to follow outlines of the fish, leaving a 1-inch border.
Crimp the edges of foil to fit up against the edge of the fish. Mix the soy sauce with the brown sugar and brush onto the salmon fillet.
Prepare the grill and add soaked wood chips for extra flavor. Lay the fish on the center of the grill. Cover and cook until the fish is barely opaque in the thickest part (cut to test), about 15-20 minutes. Transfer the fish to a platter with the salad. Add salt and pepper to taste. Serve hot or cold.