1
Make the fries: slice the potatoes thinly on a mandoline, then cut into thin matchsticks. Wash in iced water to remove starch, dry thoroughly and chill on a cloth.
2
Heat a non-stick pan until smoking, add the oil, season the steaks with sea salt and brown on all sides.
3
Reduce the heat and turn the steaks every 30 seconds; check with a metal skewer for medium-rare (warm to the touch, ~46°C). Remove and rest 5 minutes.
4
Pour the cognac into the pan (take care on gas), add the pepper and cook out 1 minute. Add the cream and stock and reduce by half, then cook on high 2 minutes to a thick sauce.
5
Return the steaks with their resting juices, coat in the sauce and season with lemon juice.
6
Heat the oil to 170°C and fry the matchstick potatoes in batches 2-3 minutes until golden. Drain, salt and serve on top of the steaks.