Preheat broiler. Arrange bread slices in a single layer on a baking sheet, and broil 1 to 2 minutes on each side or until lightly golden and toasted.
Whisk together mayonnaise, lemon zest, 3 tablespoons dill, and 1/4 teaspoon salt in a small mixing bowl. Combine crab, onion, and 1/4 teaspoon salt in a medium mixing bowl. Add 2 tablespoons mayonnaise mixture to bowl; stir to combine.
Spread remaining mayonnaise mixture evenly over toasts. Divide crab mixture evenly over toasts; top evenly with tomato slices and cheese. Broil 3 minutes or until cheese melts. Sprinkle with black pepper and remaining 1/2 tablespoon dill. Serve immediately.