Step 1Cut salmon into 3 pieces. Season with salt and pepper to taste. Wrap each piece in 2 large sorrel leaves. Tie with string to secure. Steam wrapped salmon in steamer until center of fish is still slightly red, 8 to 10 minutes. Remove from steamer. Let cool slightly, then cover tightly with plastic wrap and chill 2 hours.
Step 2Chop remaining sorrel leaves. Place in blender with mayonnaise, onion, salt and pepper to taste and blend until sorrel is chopped very fine and mayonnaise is bright green. Chill 30 minutes or until ready to serve. Remove string from chilled salmon. Serve salmon with sorrel mayonnaise.