"Love this - so good on burgers with mayonnaise, lettuce, and tomatoes. It's wonderful on a roast beef sandwich or as a low-cal veggie dip. The original recipe is of Southern origin. It came from an old handwritten farm cookbook from North Carolina. I found a jar of Cucumber Ketchup in the back of the fridge that had been in there two years. It was still delicious."
Mix cucumbers and onions in a large bowl; stir in salt until thoroughly combined. Transfer mixture to a colander, cover colander with plastic wrap, and let drain overnight.
Place drained vegetables into a large bowl and pour in apple cider vinegar; stir to combine. Working in batches if necessary, place vegetables with vinegar into a blender and puree. Pour puree into a bowl and season with mustard seed and black pepper. Spoon the ketchup into small jars with lids; cover and store in refrigerator for 3 days before serving. Ketchup keeps well in the refrigerator.