1 Remove the steaks from the fridge at least 20 minutes before cooking
2 Transfer the steaks onto a plate, brush with 10ml/2tsp of the oil on both sides, season and brush with mustard all over. Cover and leave to one side.
3 To cook the caramelised vegetables, heat the remaining oil in a large non-stick pan and cook the onions and mushrooms for 4-5 minutes under a low heat until soft. Add the sugar and balsamic vinegar. Cook for a further 5 minutes until slightly caramelised. Remove from the heat and transfer the mixture to a small bowl.
4 Meanwhile, to prepare the relish, in a small bowl mix all the ingredients together sand set aside.
5 Return the same pan and heat until hot. Cook the steaks for 2-3 minutes on each side (for medium). Slice thinly on the diagonal.
6 Cut the bread in half and spread with the relish, salad leaves, tomatoes and cucumber. Top with the steak, caramelised vegetables and more relish. Pop the other piece of bread on top, press down lightly, cut in half (if preferred) and serve with fries or potato wedges and extra relish if required.
Tips: If you require a more fiery sauce, stir through some dried chilli flakes
Recipe courtesy of Cheeky Beef