Heat your oven to 200°C/Fan 180°C/Gas 6. Put a pan of water on to boil. Scrub the potatoes and chop them into chips about as big as your fingers.
Once the water in the pan is boiling, add the chips and simmer for 5 mins. Drain and leave to steam dry in a colander for 5 mins.
Pour 1 tbsp oil onto a baking tray. Warm it in the oven for 2-3 mins, then carefully tip the chips onto the tray. Add the tomatoes. Sprinkle with salt and pepper. Cook for 30 mins till the chips are golden and crisp and the tomatoes have started to burst.
Spoon two-thirds of the butter into a small pan.
Add 1 tsp sugar and 1 tbsp balsamic vinegar. Set on a low heat. Leave to melt, stirring occasionally for 5 mins (don’t let it boil). When the caramel is thick and glossy, take off the heat and put on the lid to keep it warm.
When the chips and tomatoes have cooked for 25 mins, season the steaks. Heat a frying or griddle pan. Melt the remaining butter, then add the steaks. Cook for 1 min on each side for rare, 2 mins on each side for medium or 3 mins on each side for well done. Pop on a warm plate, cover with foil and leave to rest.
Taste the balsamic caramel. Add a touch of salt and pepper to taste. It should be thick and glossy.
Arrange the steak, chips and tomatoes on 2 warm plates. Toss the rocket with 1 tsp oil and add a handful to each plate. Pour over the warm balsamic caramel and serve.