Cut salmon in small pieces, about 1/4-inch thick. Set aside or cover and refrigerate until ready to serve.

Slice cucumber into very thin (almost transparent) rounds and set aside.

In a medium bowl, mix together the shallots, dill, chives, capers, lemon juice, and mayonnaise. Cover and refrigerate if not using immediately. Just before serving, add the tomatoes to the mayonnaise mixture and stir to combine.

On a serving plate, spread the cucumber to cover 2/3 of the plate, leaving a space in the center of the plate. Spread the mayonnaise mixture in the center of the plate and then arrange the salmon over the cucumber. Garnish with endive pieces around the edges.