Preheat oven to 350. In a medium bowl, combine the salmon, onion, parsley, capers, zest, and juice; toss. Stir in the oil; mix well. Set aside. Arrange the bread slices in a singlelayer on large baking sheet pans. Bake until almost crisp, 10-12 minutes.
Immediately spread the butter over one side of each crostini. The crostini may be baked and buttered several hours ahead. Top the crostini with the salmon mixture; sprinkle lightly with the pepper.