Heat oil in a fryer or heavy saucepan to 160°C.
Rinse chips under cold water for 2 minutes to remove starch.
Simmer chips in salted water until tender, then drain and dry.
Fry chips in batches for 8 minutes until pale golden.
Increase oil temperature to 190°C.
Fry chips again for 3 minutes until dark golden and drain.
Preheat the oven to 200°C.
Heat oil in an ovenproof frying pan until smoking hot.
Sear salted steaks for 3 minutes per side until browned.
Add butter to the pan and roast in the oven for 8 minutes.
Baste steaks with butter and rest for 10 minutes.
Serve steaks with chips and fresh watercress.