Cook the potatoes, beans and broccoli in lightly salted water until tender but still with a slight bite to them. Then plunge into ice-cold water to stop the cooking process. Drain and cut the potatoes into quarters lengthways.
Mix all the dressing ingredients together and season with salt and pepper.
Heat a griddle pan over a very high heat. Season the steaks and rub with oil. Place in the pan and sear on 1 side for about 1 minute, then turn over and sear again for a further minute (or longer if you want your tuna cooked through rather than pink).
Toss the potatoes, beans and broccoli in the dressing. Sprinkle with the hazelnuts and serve with the tuna.