Peel 2 large cucumbers.  Run the tines of a fork lengthwise down each cucumber so that you make little grooves all along the circumference.  Slice very thin into a large bowl.  Cover with salted water (a teaspoon or so to amount of water needed to cover).  Let stand for several hours in refrigerator.  Drain and add 1 very thinly sliced large onion.  

Cover with the following marinade: 
1 1/2 c. water
3/4 c. white vinegar
3/4 c. sugar
1/2 tsp. salt

Combine 4 marinade ingredients and pour over vegetables.  Let stand in refrigerator for a few hours (or a few days).  At serving time, drain.  Put in glass serving dish and sprinkle with fresh parsley.  When covered with marinade this keeps well for 2 weeks covered in refrigerator.  

Source: http://www.nancys-kitchen.com