Drain and rinse cannellini beans and put them into a bowl. Add red onion and celery sticks, drizzle with a little French dressing and season well. Divide a bag of rocket between 3 plates and spoon the bean mix over the leaves.
Brush tuna steaks with olive oil and fry on a griddle for 2-3 minutes each side. Place the tuna on top of each salad and scatter a few black olives onto each plate to serve. (Or drain a large can of tuna in oil and serve with the bean and olive salad.)