Preheat broiler. Arrange bread slices in a single layer on a 17x14-inch ungreased baking sheet. Broil 3 to 4 inches from heat for 1 to 2 minutes per side or until lightly toasted.
In a small bowl mash avocado. Stir in capers and lemon juice. Spread avocado mixture on one side of each bread slice (about 1 rounded teaspoon per slice). Layer salmon, arugula, and onion slices over avocado mixture. Sprinkle with pepper. Serve at once. Makes 24 appetizers.