Add the drained tuna to a medium bowl, and mash with a fork. Add the cottage cheese, relish, mustard, lemon zest and juice, dill, and green onion. Season with salt and pepper, to taste. Set aside.
Slice the cucumber in half, and vertically so you have four pieces. Use a spoon to scoop out the seeds of each piece, creating a “boat”.
Fill the cucumber with the tuna mixture before serving.