Place a sheet of cling film over the steaks and pound with a rolling pin until quite flat (about 5mm/¼in). Season with pepper, but do not salt.
Prepare the ingredients for the sauce now, as you won't have time later.
Heat the oil and butter in a non stick frying pan, and cook the steaks briskly for 1 minute on each side. Draw the pan off the heat, transfer the steaks to two warm dinner plates and season with sea salt.
Now, add the brandy or Cognac to the still-hot pan and return it to the heat, where - whoosh - it will flame.
When the flame dies, reduce the heat and quickly add the butter, Worcestershire sauce, mustard, sea salt and pepper, stirring vigorously with a wooden spoon for 1 minute, allowing the mixture to bubble and thicken.
Add the chives, parsley, lemon juice and any juices that have gathered from the resting steaks, stir well and pour the bubbling sauce over the steaks.
Serve immediately, with potatoes, asparagus, or a green salad.