For the white bean spread: Combine the cannellini beans, mascarpone, salt and lemon zest in a food processor. Process for 30 seconds, until a paste forms. With the machine running, add the olive oil and lemon juice; continue processing until smooth, another 30 seconds. Transfer the spread to a bowl and set aside while you make the tuna salad.
For the tuna salad: In a medium bowl, stir together the tuna, celery, fennel, fennel fronds, sun-dried tomatoes, capers, parsley, olive oil, lemon juice, salt and pepper.
To assemble: Spread each English muffin half with 2 tablespoons of the white bean spread. Add 5 or 6 arugula leaves and top with 2 tablespoons of the tuna salad. Finish with a drizzle of olive oil. Serve.