In a medium bowl, add the dijon mustard, lemon juice, salt, pepper and olive oil. Whisk to combine.
Add the tuna, cannellini beans, tomatoes, capers, arugula, and parsley. Gently mix to combine. Add salt and pepper to taste. (The capers are salty so you might just need a crack of fresh pepper.)
Serve immediately or refrigerate for up to 4 hours. Serve with crackers if desired. Enjoy!