Heat your oven to 200°C/Fan 180°C/Gas 6. Scrub the potatoes and cut them into chips the thickness of a little finger. Scatter in a roasting tin. Finely chop the rosemary leaves and sprinkle over the chips, along with a good pinch of salt and pepper. Toss with 1 tbsp olive oil and slide into the oven for 25 mins, turning halfway.
While the chips bake, peel and grate the garlic clove. Peel and finely slice the shallots. Pick the tarragon leaves off their stalks and thickly slice the chestnut mushrooms. Set aside.
Fill a pan with boiling water and add a little salt. Trim the ends off the French beans and tip into the water. Simmer for 2 mins, then drain well. Slide into a bowl and set aside. Pour 1 tbsp red
wine vinegar into a small bowl and whisk in
1 tbsp oil and a little salt and pepper. Set aside.
Bring a heavy frying pan to a high heat. Rub the steaks with a generous pinch of salt and pepper on all sides. When the pan is smoking hot, add 1 tbsp oil and the steaks. Fry for 1-2 mins on each side, then transfer to a warm plate to rest.
Reduce the heat under the empty pan to medium and slide in the shallots, mushrooms and garlic. Cook for 3-4 mins, till glossy, then add 100ml of the cream and the tarragon leaves. Stir well and bring to a simmer.
Dress the beans with the vinegar and oil. Pop the steaks onto warm plates and spoon over the mushroom sauce. Serve straight away with the rosemary chips on the side.