To make the "tuna", drain the chickpeas and rinse them well. Add them to a bowl and mash with a fork or potato masher. They can be fairly chunky still, but you shouldn't have any whole chickpeas left.
Now add the vegan mayo, Dijon mustard, salt, lime zest and chopped dill and mix with the chickpeas.
Heat up a fry pan and add half a nori sheet. Toast on both sides until you can smell the nori. This should only take about 1 minute. Once the nori is toasted, crush it up into small pieces (with your hands or a mortar and pestle) and add them to the chickpeas. Mix.
To combine the sandwich: Toast the bread slices and butter them if desired. Add the chickpea mixture onto the toast, then some cucumber slices and top it of with fresh watercress before closing the sandwich with the second slice of bread. Enjoy!