Using a vegetable peeler, peel the cucumber, leaving about 1/2 inch between peels so that the cucumber skin stays on in alternating stripes. Cut off both ends of the cucumber, then slice into 1/2-inch thick chunks. Scoop out the center of each slice so that there's a little divot for the tuna salad. Put on a plate and set aside.

Using a spatula, empty tuna into a medium sized bowl and break up any large chunks. Add mustard and mayo and toss to thoroughly coat tuna. Add celery, bell pepper and paprika and mix well.

Spoon a heaping tablespoon on top of each cucumber slice. Refrigerate any leftover tuna salad and serve cucumber cups immediately. Cucumber cups can be made ahead and refrigerated overnight, but you may have to drain the plate in the morning, as the cucumber will release moisture overnight.