Gently fry the onion in a little of the olive oil till soft and caramelised. This should take 10-15 minutes
Meanwhile, beat the steak a little with a meat tenderiser or a rolling pin so that it is no more than 1/2cm thick. Rub all over with olive oil and season well with pepper and sea salt flakes.
Heat an iron griddle over a medium heat. Once the onions are cooked, put them in a warm oven to keep hot.
Sear the steak for 1-2 minutes on each side then remove from the pan and allow to rest in a warm oven
Add any remaining oil to the griddle, then toast the sourdough bread on both sides
Slice the steak into finger sized strips
Spread mustard on one side of each slice of toast, then layer onion, steak, chilli jam, lettuce and tomato onto one slice and top with the second slice.
Turn the whole thing over and, if you like cut into half and pin with a cocktail stick.
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