Brush tuna with olive oil. Heat a medium-sized non-stick frying pan and cook the tuna steaks for 2 minutes on each side, or a little longer if you prefer them cooked through. Remove from pan, set aside and keep warm. Add a glug of olive oil, pan-fry the caper berries for about a minute, and set aside. Place 3 polenta wedges on each plate, top with a tuna steak and a handful of rocket, and scatter with pan-fried caper berries. Season well and serve.