@flatironsteak: Smashed Onion Double Cheeseburger - now available at all Flat Iron restaurants! Two smash beef patties from our herd in Thirsk, slices of sweet onion smashed into the patties, American cheese, sliced pickles, American mustard, ketchup bearnaise and topped with crispy batter scraps for extra crunch. #burger #flatiron #london #restaurant #steakrestaurant