Drain mushrooms; reserve liquid, and set aside.
Melt 2 tablespoons butter in a heavy saucepan over low heat; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add whipping cream and reserved mushroom liquid; cook over medium heat, stirring constantly, until mixture is thickened and bubbly. Stir in salmon and mushrooms. Continue to cook until thoroughly heated.
Spoon creamed mixture evenly into patty shells. Garnish each with watercress; serve immediately.
Note: Cooked chicken, tuna, or any fish combination may be substituted for salmon.