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 finally add the salmon. Cover and refrigerate if not using immediately. Just before serving, chop the tomatoes, season with salt and pepper and a sprinkling of olive oil, and stir to combine. On a serving plate, spread the cucumber slices to cover 2/3 of the plate, leaving a space in the center of the plate. Then arrange the salmon over the cucumber, then top with the seasoned tomatoes. Garnish with sliced avocado and chopped chives. Serve with endive, toast points or crackers.