For the chilli butter, mash the parsley into the butter with the chilli, a squeeze of lemon juice, the zest and some seasoning. Chill until firm.
Heat oven to 200C/180C fan/gas 6. For the chips, toss all the ingredients on a baking tray, then season. Roast for 35-45 mins until golden and crisp.
Heat a good glug of oil in a frying pan. Add the extra butter and the seasoned steaks, fry for 2-4 mins on each side, then transfer to plates. Toss the leaves with olive oil and lemon juice. Divide between the plates, add the chips and top each steak with a piece of chilli butter.