Thaw fish, if frozen. Measure thickness of fish fillets. Add 1-1/2 cups water to a large skillet. Bring to boiling. Add fish fillets. Return to boiling; reduce heat. Simmer, uncovered, about 4 minutes per 1/2-inch thickness, or until fish flakes easily when tested with a fork. With slotted spatula, lift salmon to a plate lined with several thicknesses of white paper toweling. Allow salmon to cool for 15 minutes. Remove paper toweling; cover salmon with plastic wrap and chill for 4 hours.

Combine mayonnaise, yogurt, onion, capers, lemon peel, and salt; set aside. Remove any dark flesh from chilled salmon. Flake salmon and add to the mayonnaise mixture; stir with a fork to combine.

Spread each slice of bread with some of the salmon salad. Top half of the slices with watercress. Add remaining slices of bread; spread-side down. Cut sandwiches in quarters to form squares, rectangles, and/or triangles. If desired, top each piece with an olive slice and a decorative pick. Makes 48 bite-size sandwiches (8 servings).

To make your sandwiches a little more festive and fun for a card party, cut the bread before making the sandwiches, using small (about 2-inch) cutters in the shapes of hearts, spades, diamonds, and clubs. For these shapes, you will need extra slices of bread.

Salmon can be poached and chilled one day ahead. Dressing can be made one day ahead, covered, and chilled. Flake the salmon and add it to dressing when ready to assemble sandwiches. Sandwiches can be filled and refrigerated, covered tightly with plastic wrap, for two hours before serving.