How to Make It
In a small bowl, mash the butter with both mustards and the kosher salt until evenly combined. Spread some of the butter on 1 side of each slice of bread. Top 4 of the buttered sides with the cucumbers and onion and season with flaky salt and several grinds of black better. Sandwich with the remaining 4 slices of bread, mustard-butter side down. Cut off the crusts if you'd like and cut each sandwich into 4 squares, rectangles, or triangles. For extra credit, punch out shapes with cookie cutters (circles and stars work best—witches on broomsticks, not so much). Arrange the sandwiches on a platter and serve.