Flank Steak with Mushrooms:
450 g (1 lb) flank steak, at room temperature
sea salt and freshly ground black pepper
2 Tbsp extra virgin olive oil
20 g (3/4 oz) butter
2 French shallots, finely chopped
350 g (12 oz) pine mushrooms (or Swiss brown and flat mushrooms), thickly sliced
a few thyme sprigs
micro herbs, such as red cabbage cress or sorrel, to serve
Sweet Potato Crisps:
2–3 small orange sweet potatoes, peeled
1 Tbsp rice bran oil
1/4 tsp fine sea salt
1 1/2 tsp ground sumac
1 tsp flaked sea salt
For the sweet potato crisps, preheat the oven to 200°C (400°F). Lightly grease two large baking trays with oil.
Use a mandolin to slice the sweet potato into very thin, almost transparent rounds.
Put the sweet potato in a bowl with the oil, fine sea salt and sumac, and toss to coat completely. Arrange the slices on the baking trays in a single layer, being careful not to overlap them. Sprinkle over the flaked sea salt and bake for 5–6 minutes, or until they are slightly golden brown with a little give in the centre (they will firm up).
Check them regularly as they cook to make sure they don’t burn, and turn them with tongs halfway through cooking. Depending on the size of your oven, you may need to bake them in a few batches. The crisps will keep for up to 15 hours, stored in an airtight container (the fresher they are, the crisper they will be).
Heat a large frying pan over high heat until smoking. Season the steak with salt and pepper and rub over half the olive oil. Add the steak to the hot pan and sear for 4 minutes on each side, or until a crust forms. Transfer to a plate to rest for 5–10 minutes.
Reduce the heat to medium–low, add the remaining oil and the butter to the pan and fry the shallots for 5 minutes, or until softened. Add the mushrooms and cook until golden brown, being careful not to overcrowd the pan. Add the thyme sprigs and cook for a further 1–2 minutes.
To serve, put steak on a serving board and cut across the grain into 2 cm (¾ in) thick slices. Spoon the mushrooms over the top with any remaining pan juices. Scatter over micro herbs and serve with sweet potato crisps.