Steam broccoli in a colander over simmering water or in the microwave until tender-crisp. Set aside.
Meanwhile, in a medium bowl, whisk together apple, yogurt, mayonnaise, mustard, and horseradish. Arrange salmon on a serving platter. Arrange black bread slices alongside and garnish with lemon wedges and capers. Serve half of the broccoli with this meal and reserve remaining broccoli (about 2 cups) for another recipe.