How to Make It
In a large pot, bring about 1 inch of water to a boil. Put the fish in a large steamer basket, skin-side down; sprinkle with 1/4 teaspoon of the salt and 1/4 teaspoon pepper. Pile the watercress on top. Sprinkle with another 1/4 teaspoon salt.
Put the steamer basket with the fish over the boiling water and cover. Reduce the heat to moderately high and cook until the watercress is wilted and the salmon is just barely done (the fish should still be translucent in the center), about 5 minutes for a 1-inch-thick fillet.
Meanwhile, in a small stainless-steel saucepan, melt the butter. Add the lemon juice, the remaining 1/4 teaspoon salt, and a pinch of pepper. Serve the salmon and watercress topped with the lemon butter.
Fish Alternative Use halibut fillets instead of salmon. Steaks from either fish will work, too; a one-inch-thick steak takes the same time to cook.
Removing Pin Bones: Many types of fish fillets have a line of small bones running the whole length just to one side of the center. Run your finger down the fillet to find them. The best way to remove these bones is with long-nosed pliers. Don't worry if you don't have the time, though. The bones are much easier to remove as you eat, when the flesh is cooked.