Heat your oven to 200°C/Fan 180°C/Gas 6. Fill a pan with water. Add a pinch of salt. Cover and bring to the boil. Peel the potatoes. Slice them into chips about 1 cm thick. Add to the boiling water.
Bring the water back to the boil. Simmer for 3 mins. Drain the chips. Tip into a large roasting tin or baking tray. Drizzle over ½ tbsp oil and mix. Bake for 25-30 mins till golden, turning once or twice.
Rub the tomatoes with 1 tsp oil and season. Pop them on a baking tray or in a small roasting tin. After the chips have been cooking for 15 mins, put the tomatoes in the oven to roast for 15 mins.
Slice the mushrooms. Peel and crush the garlic. Pick the leaves off half the rosemary sprigs. Finely chop the leaves.
Pick off the large watercress stalks. Pop the watercress in a bowl. Drizzle with 1 tsp oil and season with salt and pepper.
Season the steaks. Heat a griddle or frying pan till it’s smoking hot. Add half the butter and melt. Carefully add the steaks to the hot pan and fry for 1-2 mins on each side if you like it rare, 2-3 mins on each side for medium and 4-5 mins on each side for well done.
Pop the steaks on a board or plate and wrap them in foil. Leave them to rest. Add the remaining butter to the pan and swirl to melt it. Add the mushrooms. Season. Stir in the garlic and rosemary.
Fry the mushrooms over a high heat for 5 mins till golden. Turn the heat down. Add the cream. Stir and cook for 2 mins till the sauce is warm. Taste and adjust the seasoning. Serve the steaks with the chips, watercress, roast tomatoes and mushroom sauce.