01 - In a medium mixing bowl, add the sliced cucumber, chopped red onion, and fresh dill.
02 - In a separate bowl, whisk together the mayonnaise, Greek yogurt, lemon juice, Dijon mustard, salt, and black pepper until smooth.
03 - Pour the prepared dressing over the cucumber mixture and toss gently to coat evenly.
04 - If desired, spread a thin layer of softened unsalted butter on one side of each bread slice.
05 - Spoon the cucumber salad evenly onto four slices of bread, then top each with the remaining slices to close the sandwiches.
06 - Slice sandwiches in half and serve immediately.