Onglet steak with watercress butter and shoestring potatoes

Method

1. Heat the oven to 220°C/200°C fan/gas 7. Put the butter in a food processor with 80g of the watercress, the garlic, lemon zest and a pinch each of salt and pepper, then whizz until combined. Lay a sheet of cling film on the work surface, then spoon the butter along one edge. Roll up into a log, twist the ends to seal, then chill until needed.
2. Spread out the potato strands on a large oven tray (or 2 smaller trays) and toss with the rosemary and 2 tbsp olive oil. Sprinkle with salt, then bake for 10 minutes. Toss using a sturdy spatula (the potatoes can stick), then bake for 3-5 minutes more until some are completely crisp and others are softer. Finely chop 20g of the remaining watercress, then toss with the potatoes along with the parsley and parmesan.
3. For the steak, heat a griddle pan over a high heat until smoking hot. Brush the steaks with a little olive oil and season well. Cook for 2½ minutes on one side, pressing down with a spatula to get a good colour, then flip and cook for another 2 minutes for medium-rare or until done to your liking. Transfer to a board and top with a couple of sliced discs of the watercress butter, then cover lightly with foil and leave to rest for 5 minutes before slicing. Serve with the shoestring potatoes and the rest of the watercress, dressed with a little olive oil.
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