Prepare the ingredients as instructed.
Add the cucumber slices to the water and refrigerate for at least 10 minutes. Serve over ice if desired.
Bring the water and oil to a boil. Cook the couscous as instructed on the package.
In a large bowl, whisk together the olive oil, lemon juice, lemon zest, Dijon mustard and black pepper until well combined.
Add the cannellini beans and gently mash about one-third of the beans with a fork to help bind the salad.
Flake the tuna into the bowl, then add the red onion, parsley, capers, and red pepper flakes, if using. Fold everything together gently, keeping the tuna in large chunks.
Let the salad rest for 10 minutes to allow the flavors to develop.
Portion the couscous, top with arugula and tuna and white bean salad. Serve with cucumber and radish slices for scooping, cottage cheese, apple slices, and cucumber water.