Thinly peel the cucumbers, cut them in half lengthwise and scoop out the seeds.
Cut the cucumber halves crosswise into slices about 1/4 inch thick.
Put the slices into a large mixing bowl and sprinkle lightly with salt.
Cover and let stand for at least 1 hour (the salt extracts the excess water, and crisps the cucumber).
Rinse the cucumber and drain well in a colander or pat dry with paper towels.
Put the cucumber, fennel and chopped scallions into a large salad bowl.
Add the yogurt and seasoning.
Mix together until well mixed.
Cover the salad and refrigerate until ready to serve.