01 - Place sliced cucumber, chopped red onion, and chopped dill in a medium mixing bowl.
02 - In a separate mixing bowl, whisk together mayonnaise, Greek yogurt, lemon juice, Dijon mustard, salt, and black pepper until smooth.
03 - Pour the dressing over the cucumber mixture and toss gently to ensure all ingredients are evenly coated.
04 - If desired, spread a thin layer of softened butter on one side of each bread slice using a spoon or knife.
05 - Spoon cucumber salad evenly onto 4 slices of bread. Top each with a remaining bread slice, buttered side inward if used, to form sandwiches.
06 - Cut sandwiches in half with a sharp knife and serve immediately for optimal freshness.