These high-protein "sandwiches" are best assembled and served right away. We use English cucumbers because the skin is thinner than the hothouse variety and doesn't need to be peeled. Be sure to serve each sandwich wrapped in wax paper to keep all the juices contained--then bite with confidence!
Halve the cucumbers lengthwise and use a spoon to scoop out the seeds and some of the flesh to make a boat shape; discard seeds and flesh. Poke the inside of the hollowed out cucumbers several times with a fork. Whisk together the vinegar, mustard, 1/8 teaspoon salt and a few grinds of pepper in a medium bowl. Slowly drizzle in the oil, whisking constantly, until blended. Brush the inside of each cucumber with some of the vinaigrette.
Whisk the yogurt, dill, celery leaves, mayonnaise, scallions, celery, lemon zest, 1/4 teaspoon salt and a few grinds of pepper into the bowl with the remaining vinaigrette. Add the drained tuna and stir to combine.
Assemble the sandwiches: Fill two of the cucumbers with the tuna salad and top each with 1/4 cup of sprouts. Sandwich with the remaining cucumbers. Cut each sandwich in half and wrap in wax paper (this will help catch all the juices).
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