Wash and dice the English cucumbers into small, bite-sized pieces. Sprinkle with salt and let sit in a colander for 15 minutes.

In a mixing bowl, combine yogurt and crumbled feta cheese. Stir in chopped fresh herbs, lemon zest, and juice. Season with black pepper and mix until incorporated.

After draining, pat cucumbers dry with paper towels and fold them into the yogurt-feta mixture.

Toast the bread until golden brown. On one slice, layer sprouts, then spread the cucumber mixture. Add onion slices if desired, and top with another slice of bread.

Cut sandwiches in half diagonally and serve immediately.