For the sauce: whisk the fish sauce, lime juice and brown sugar until the sugar has melted then add fresh garlic, chili, coriander and chopped red onion.
Clean the beef, removing all string and some excess fat and marinate with the oyster sauce.
Marinate the cucumber, sliced onion, cherry tomato, spring onion with salt and squeeze half of the fresh lime.
Pan-sear the beef until it’s cooked ‘medium’ and slice it.
Plate the beef and decorate with the cucumber, sliced onion, cherry tomato and spring onion.