Line a rimmed baking sheet with paper towels and lay the cucumber slices in a single layer. Sprinkle lightly with salt and let sit for 20 minutes, then pat the slices dry with additional paper towels. Taste one slice and season with a little more salt if needed.

Arrange the bread slices on a cutting board. Generously spread one side of every slice with the softened butter or cream cheese, using 1 to 2 teaspoons per slice depending on the size of the bread.

Place the cucumber slices on half of the bread slices, overlapping them slightly so the bread is completely covered. If the slices are very thin, layer them twice; if thicker, a single layer is enough. The filling should be about 1/4 inch thick. Dust the cucumbers with the pepper and, if you like, scatter on watercress, fresh mint, Meyer lemon zest, dill, chives, or parsley. Cap each sandwich with a second slice of bread, buttered side down.

Press the sandwiches down gently but firmly so the layers hold together. Trim off the crusts with a sharp serrated knife, then slice each sandwich into rectangles, triangles, or quarters for neat, bite-size pieces.

Transfer the finished sandwiches to a serving platter and serve immediately, or keep them covered in the refrigerator until ready to serve.