Sandwich the fillet tail pieces between two sheets of cling film. Using a rolling pin, flatten the steaks to a 1cm/½in thickness.
Mix the ground coriander, chilli flakes, peppercorns, oregano, mace, ground bay leaf, garlic, salt and vegetable oil in a large bowl until well combined.
Place the flattened steaks in the herb mixture, cover and set aside to marinate for as long as possible.
Heat a large heavy based frying pan over a high heat. When the pan is smoking hot, sear the steaks on one side for one minute, or until they start to blacken. Turn the steaks over and add the butter to the pan, then fry for a further minute.
Place two pieces of steak into each buttered bread roll. Serve immediately.