Preheat the oven to 220 degrees celsius.
Halve your potatoes (cut larger ones into thirds) then place in a large oven proof dish. Drizzle with 2 tablespoons of the olive oil, then place in the oven for 30 minutes. After 30 minutes, the potatoes should be soft and starting to turn golden. At this point, add your roughly chopped courgettes and put back in the oven for another 10 minutes. Add the vine tomatoes for a final 10 minutes.
Close to the end of the cooking time for the veg, drizzle your tuna steaks with the remaining olive oil, and sprinkle with a little salt. Get a griddle pan to a very high heat, then add the steaks. Cook for 2 minutes on each side (they should still be lovely and pink in the middle).
To serve, dish up your veg. Drizzle a little balsamic vinegar over it and sprinkle over some chopped parsley. Serve with your tuna steak on the side. Enjoy!