Cold Poached Salmon Steaks with Watercress Sauce

1 medium lemon
4 salmon steaks, 1 inch thick (6 ounces each)
3⁄4 teaspoon salt
1⁄2 teaspoon coarsely ground black pepper
1 medium onion, thinly sliced
Watercress Sauce (see below)

1. From lemon, squeeze juice; reserve for Watercress Sauce. Set lemon shells aside. Rub salmon steaks evenly with salt and pepper.
2. In 12-inch skillet, heat 1⁄2 inch water to boiling over high heat. Add salmon, onion, and lemon shells; heat to boiling. Reduce heat; cover and simmer until fish is just opaque throughout, 5 to 8 minutes. With slotted spatula, transfer fish to platter. Let cool 30 minutes, or cover and refrigerate to serve later.
3. Meanwhile, prepare Watercress Sauce.
4. Remove skin and bones from salmon, if you like. Serve with sauce. Makes 4 main-dish servings.

Watercress Sauce
In blender or in food processor with knife blade attached, puree 1⁄2 bunch watercress, tough stems trimmed (1 cup), 1⁄2 cup sour cream, 1 tablespoon fresh lemon juice, 1 teaspoon chopped fresh tarragon or 1⁄8 teaspoon dried tarragon, 11⁄2 teaspoons sugar, and 1 teaspoon salt until smooth. Cover and refrigerate. Makes about 1⁄2 cup sauce.