Dice the English cucumbers into small, bite-sized pieces, sprinkle with salt, and place them in a colander for 15 minutes to draw out excess moisture.

In a mixing bowl, combine Greek yogurt, crumbled feta cheese, chopped fresh herbs, lemon zest, lemon juice, and a pinch of black pepper; mix until smooth.

Fold the drained cucumbers into the yogurt-feta mixture in the bowl.

On one slice of toasted bread, layer sprouts and then spread the cucumber salad mixture evenly over it. Add onion slices if desired, then top with another slice of toasted bread.

Cut the sandwiches in half diagonally and serve immediately.