Heat your oven to 200°C/Fan 180°C/Gas 6. Scrub the sweet potatoes and slice them into around 8 wedges each.
Pop in a bowl with ½ tbsp olive oil, 1 tsp smoked paprika and some salt and pepper. Toss to coat in the oil. Spread out on a baking tray and bake for 30-35 mins.
Tip the porcini into a small food processor, spice or coffee grinder and whizz to make a fine powder (see our tip if you don’t have one). 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 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 marjoram leaves. Stir the garlic, shallot and chopped marjoram into the butter. Put the bowl in the fridge.
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.
Serve the steaks topped with a spoonful of the herb butter, the sweet potato wedges and a handful of watercress. Garnish with a few marjoram leaves. (Any leftover herb butter will keep for a few days in the fridge or frozen for up to 3 months.)