I've been making a version of this that I found in Sunset Magazine as Tuscan Tuna Salad a few years back. It adds celery and leaves off the rosemary, arugula and lemon juice. I buy tuna packed in olive oil at Trader Joe's Market and it seems to have just the right amount, without needing to drain and add oil back in. I have used a 1/2 can and a whole can of white beans, at various times. Either way seems to work. I haven't tried the idea of mashing part of the beans. That is very Italian. I will try that next time.
The lemon juice makes this so fresh; the rosemary lends an earthy flavor; the arugula has a crisp, peppery bite; and the beans and tuna make this so hearty for my lunch. I served it with Black Pepper and Olive Oil Triscuits. Yum.