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Season the steak well, then fry it with the garlic in hot oil for 4-5 mins on each side for medium rare. Put the steak on a board to rest and discard the garlic.
Add the peppers and a pinch of salt to the pan with extra oil if needed and cook for 3-4 mins.
Heat grill to high. Thinly slice the steak. Fill each bun with a quarter of the steak, peppers and cheese slices. Grill for 2-3 mins, drizzle with mustard and top with onions.