1. Preheat the oven to 230°C, gas mark 8. Lay the steaks out on a board and bash with a rolling pin or smooth meat mallet until about 1cm thick. Spray with olive oil, season, then leave to come up to room temperature.
2. Place the frites into the oven and cook according to pack instructions. Put the rosemary salt ingredients into a mini processor and blitz briefly to chop finely (or finely chop with a sharp knife). Set aside in a bowl.
3. Place all the mustard butter ingredients into a clean food processor and blitz to combine. Spoon onto a piece of clingfilm, then wrap and roll into a squat 4cm long cylinder. Twist the ends like a cracker and place into the freezer for about 10 minutes.
4. Meanwhile preheat a griddle pan for at least 5 minutes or until smoking hot. Add the steaks and cook for a minute on each side, pressing them down flat with a fish slice as they cook. Place onto warmed plates. Cut the chilled butter into 4 even slices and place on top of the hot steaks. Toss the frites with the rosemary salt until coated. Serve with steamed green beans or a dressed salad.