Peel and cut cucumbers in ¼" slices. Slice onions thinly. Place in bowl and sprinkle with salt. Cover; refrigerate overnight.
Drain off any liquid. Chop cucumbers and onions.
Combine vinegar and sugars. Blend together mustard, turmeric and cornstarch. Add enough vinegar mixture to make a thin paste. Stir paste into vinegar mixture. Bring to a boil, stir