1. Cut ½ inch off the tip of each cucumber, then peel the cucumbers with a vegetable scraper or sharp knife. Cut out the seeds and center pulp with a long iced-tea spoon, leaving a shell about ¼ inch thick.
2. Pour ¼ teaspoon of salt into each cucumber, rubbing it in evenly with your forefinger, let the shells stand about 15 minutes, then dry them inside with a piece of paper towel.
3. In a medium-sized mixing bowl, mash the sardines to a paste with a fork or wooden spoon. Add the ham, eggs, onions, pickles, mustard and 2 tablespoons of mayonnaise.
4. Stir the ingredients together until the mixture holds its shape in a spoon. (If it seems too dry, add more mayonnaise.) Taste for seasoning. The amount of salt needed will depend on the saltiness of the sardines and ham.
5. Stuff the cucumbers by standing them on end and spooning the filling in, tamping it down with a spoon as you proceed. When they are all tightly packed, wrap them separately in wax paper or aluminum foil and refrigerate them for 2 hours, or until the filling is firm.
6. To serve, slice the cucumbers crosswise, on a slant, in slices about ½ inch thick.
