My mother makes this. I'm not sure whether you consider it a soup or a salad.
You want to use pickling cucumbers, not those dark green waxed cukes you typically find in chain grocery stores. I don't know what those are; but where I come from, they aren't considered to be cucumbers.
Use three to four medium-sized cucumbers. Cut off and discard each end. As I make any fresh cuke dishes, I always taste the blossom end tip (which I would discard). Sometimes you will find a particularly bitter cuke. These I discard. Peel them.
Slice into as thin of slices as possible. Use a small glass bowl. Chop up a small onion and add. Also add 1/4 tsp. salt and 1/2 tsp. black pepper. Add enough milk to cover cukes and onions. Cover and refrigerate. This is best if it has a couple hours to marinade.