Gently heat the oil in a dutch oven or large saucepan on medium heat and add garlic. Saute until slightly golden and add stock, fish sauce, oyster sauce, soy sauce, lime juice, chilli flakes and ginger.
Bring to a gentle simmer and allow to cook, covered, for 30 minutes. Season to taste with salt and pepper. Meanwhile, slice steak into paper thin strips and set aside.
Submerge noodles and pak choi in the broth snd cook for 5 minutes or until pak choi is tender. To serve, divide noodles pak choi into six bowls and add raw beef on top. Ladle hot broth over beef to cook through. Sprinkle sesame seeds over top and serve.
