Prepare the ingredients as instructed.
Mix the water and cucumber slices, refrigerate to infuse.
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 flavours to develop.
Portion the arugula and spoon the tuna and white bean salad over the top. Serve with cucumber planks and radish halves for scooping, cottage cheese, apple slices, and cucumber water.