Spread the cucumber slices in one layer on a cutting board or other surface. Sprinkle liberally with salt. Allow to sit for 20 - 30 minutes, then gather up the slices in a clean towel or paper towel and squeeze as much liquid as possible from the cucumbers.
Meanwhile, mix a sugar water solution of 1/2 tsp sugar per cup of cold water. Soak the onion slices in the sugar water for 15 - 30 minutes and drain well. Pat dry with a paper towel.
Mix the vinegar and oil in a 1:5 solution and whisk to emulsify. Arrange the onions on the plate, then top with cucumbers and drizzle sparingly with the dressing mixture.