For the pickled mushrooms, put the mushrooms in a heatproof bowl. Bring the other ingredients to the boil and pour over the sliced mushrooms.
For the oyster emulsion, put the oysters in a blender and blend until smooth, slowly adding the oil. Season with lemon juice and salt.
For the beef, heat frying pan until hot and add the oil. Once hot, seal the beef on each side for 1 minute. Transfer to a plate and allow to rest. Add the XO sauce to the pan and deglaze, scraping the cooked on parts from the pan into the sauce.
To serve, cut or break the bun in half and assemble the bun with the steak, sauce, mushrooms, emulsion, lettuce, mint and basil with the lid on top. Garnish with oyster leaves and edible flowers.