Slice cucumbers 1/4 in. thick. Slice onions in thin slices (should be about 4 1/2 cups). Place in microwave proof bowl. Mix the salt, mustard seed, sugar, white vinegar and celery seed. Pour this mixture over cucumbers. Microwave 3 minutes on high – stir, plus 3 minutes on high – stir, plus 3 minutes on high. Cool and keep in refrigerator in pint canning jars if you wish. Can be eaten same day.