Place the diced cucumber in a medium mixing bowl and stir it with the salt. Allow the mixture to sit for approximately 10 minutes so the salt can draw out some of the cucumber’s excess liquid.
In a separate large bowl, blend the Greek yogurt with the crumbled feta, chopped herbs, lemon zest, lemon juice, and black pepper. Stir until the mixture becomes evenly combined and creamy.
Once the cucumber has rested, drain away any released liquid. Transfer the cucumber pieces to a paper towel and gently pat them dry to remove as much moisture as possible.
Add the dried cucumber to the yogurt and feta mixture. Fold thoroughly until the cucumber is evenly coated in the seasoned yogurt dressing.
Lay out two slices of whole-grain bread. Divide the alfalfa sprouts evenly between the slices, then spoon the cucumber salad mixture over the sprouts.
Place one thin slice of red onion over each cucumber layer. Cover each sandwich with the remaining slices of bread.
Cut the sandwiches in half to secure and portion them neatly. Serve immediately while the bread is fresh and the filling is cool, crisp, and creamy.