Begin by dicing one English cucumber into small cubes, then place them in a bowl. Sprinkle kosher salt over the cucumbers and let them sit for 5-10 minutes.

Transfer the cucumbers to a fine mesh colander and gently press down to remove excess moisture.

In a mixing bowl, combine plain Greek yogurt, crumbled feta cheese, fresh lemon juice, lemon zest, chopped fresh dill, and black pepper. Stir together until well blended.

Fold the drained cucumber cubes into the yogurt mixture gently, ensuring all cucumber pieces are evenly coated.

Spread a thin layer of softened unsalted butter on one side of each slice of bread.

On the unbuttered side of one slice of bread, spread the cucumber salad filling. Top with another slice of bread, buttered side up, and press gently together.

Trim the crusts from the edges and cut the sandwiches into triangles or quarters before serving.