1. In a small bowl, combine eight ounce cream cheese, red onion, capers, dill, and lemon juice; season with salt and pepper, and stir until smooth. Lay four bread slices on a clean work surface; trim 1/4 inch from all sides of slices. Spread 1 tablespoon cream-cheese mixture on top of one slice, and layer with smoked salmon and cucumber (try not to overlap cucumber slices). Spread another thin layer of the cream-cheese mixture on top. Repeat with two more bread slices.

2. Neatly stack prepared slices, filling side up, squaring the sides with your hands; top with the fourth bread slice. Repeat with remaining bread and filling to create three more stacks in this way.

3. Transfer stacks to a serving platter with their long sides touching. In a small bowl, combine remaining 16 ounces cream cheese and creme fraiche until smooth. Using offset spatula, spread mixture evenly over assembled cake. Refrigerate, covered, two hours or overnight.

4. Before serving, arrange overlapping cucumber slices around base of cake; spoon salmon roe and caviar along the top, and sprinkle with dill. Garnish the platter with lemon wedges.