Heat your oven to 200°C/Fan 180°C/Gas 6. Scrub the sweet potatoes and slice them into chips about as big as your little finger.
Pop in a bowl with ½ tbsp olive oil and some salt and pepper. Toss to coat in the seasoned oil. Spread the chips out on a baking tray and bake for 30-35 mins till tender and caramelised.
Tip the porcini mushrooms into a small food processor, spice or coffee grinder and whizz to make a fine powder (see our tip if you don’t have a gadget to help you out). Tip the porcini dust into a bowl and add ½ tbsp olive oil and some salt and pepper. Stir to make a thick paste.
Smear the porcini paste onto both sides of the steaks. Set aside to marinate.
Scoop the butter into a bowl. Peel and very finely chop the shallot and garlic. Finely chop most of the parsley leaves. Stir the garlic, shallot and chopped parsley into the butter. Set aside.
Warm a griddle or frying pan over a high heat. When it’s smoking hot, add the steaks. Fry for 3-4 mins on each side for medium-rare, or longer if you prefer your steaks more well-done. Slide them onto a warm plate. Cover with foil. Rest for 5 mins.
While the steaks rest, scoop the garlicky butter into the pan and sizzle for 1-2 mins to melt it.
Serve the steaks with spoonfuls of the herb butter drizzled over the top, along with the sweet potato chips and a handful of watercress. Garnish with a few fresh parsley leaves.