Preheat the oven to 180C/350F/Gas 4.
For the steak, rub the oil into the meat and season well with salt and freshly ground black pepper. Heat a chargrill pan until smoking and griddle the steak for 1-2 minutes on both sides (for rare), or for longer if you prefer your steak medium or well done. Add the butter to the pan during the last minute of cooking and use to baste the meat. Transfer to a warm plate to rest.
For the prawns, heat the olive oil in the same pan and fry the prawns for 1-2 minutes on both sides, or until pink and cooked through.
For the tomato, drizzle the cut sides of the tomato with the oil, place on a baking tray and roast in the oven for 10-12 minutes, or until tender.
To serve, place the steak on a serving plate and arrange the prawns and roasted tomato alongside.