Cut the potatoes into easy to eat pieces, and boil in slightly salted water.
When the potatoes cooked through drain them well, add the vinegar (or lemon juice) while still hot, and return the the pan and shake around to distribute the vinegar.
Slice the onion with a slicer, and put into a bowl of water.
Slice the zucchini with a slicer.
Bring a pot of lightly salted water to a boil and put in the zucchini slices. When it comes to a boil again drain and put the zucchini in cold water, and drain again immediately
Squeeze out the onion and zucchini tightly to expel as much moisture as possible. Chop up the capers if you are using them. Drain the oil from the tuna.
Mix the potatoes, onions, zucchini, tuna, mayonnaise, grainy mustard, maple syrup and capers in a bowl.
Adjust the seasoning with salt and pepper to finish.
Story Behind this Recipe
I came up with this recipe when I was living in Canada. The cucumbers sold there were a bit different from Japanese cucumbers, so I started adding other vegetables instead. I add a tiny bit of Canadian maple syrup as a secret ingredient.
If you add a small amount of capers, the flavor will really come together. (You can omit them if you like though.) Just a little drizzle of maple syrup (or honey) also makes a difference. It has a much ricer flavor than sugar.