Dice the cucumbers into small cubes, sprinkle with salt, and allow to sit for 10 minutes.

In a mixing bowl, combine yogurt, feta, herbs, lemon zest, lemon juice, and pepper.

Drain the cucumbers in a colander and pat dry with paper towels.

Fold the drained cucumbers into the yogurt mixture until well coated.

Toast the bread slices until golden brown, then allow to cool slightly.

Spread cucumber filling on one slice of toasted bread, add onion if desired, and top with another slice.

Slice the sandwiches in half and serve immediately.