For the chips, peel the potatoes and cut into whatever size you prefer. Wash well in cold water until the water runs clear, drain and pat dry with a clean tea towel.
Fill a deep-fat fryer or a large, deep, heavy-bottomed pan two-thirds full with vegetable oil. Heat the oil to 140C-160C/285F-300F (use a probe thermometer to check the temperature if you're not using a deep-fat fryer). (CAUTION: Hot oil can be dangerous. Don't leave unattended)
Carefully place the chips in the hot oil and cook for 7-8 minutes or until just cooked through (without colouring). Remove from the oil using a slotted spoon, drain onto kitchen paper and set aside to cool. You may do this a few hours in advance, keeping the chips covered, in the fridge.
Meanwhile, to finish the chips, deep-fry the chips in batches again in the oil at 180C/350F for two minutes, or until crisp and golden-brown. Drain on kitchen paper and season lightly with salt.
For the steak, cut small slits across the line of fat on the steak and drizzle a little oil over the fat. Heat a griddle pan over a high heat. Salt the edge of the fat on the steak and place it onto the pan, standing it up on the fatty edge. Hold it there for a few seconds until the fat begins to run down the grooves of the grill; this is all the lubrication you will need to cook the steak
Allow the steak to fall over, season it very lightly with salt and plenty of pepper, then grill for two minutes on each side for a rare steak; three minutes for medium; and four minutes for well-done. However, remember the thinner the steak, the less the cooking time.
Once cooked, leave the steak to rest on a warm plate in a warm place (such as a very low oven) for a few minutes, so that the juices will settle evenly within the meat, while also allowing the meat to relax and become even tenderer.
Serve the steak and chips with Dijon mustard.