For the broth, heat the olive oil in a pan and fry the chilli and garlic for one minute.
Add the soy sauce, lime zest and vegetable stock and bring to the boil.
Add the shiitake mushrooms and the rice noodles and reduce to a gentle simmer. Cook for a further three minutes or until tender.
Add the choy sum and lime juice and gently simmer for a further two minutes.
Pour into a bowl and keep warm.
For the crispy noodle cake, heat the oil in a heavy based saucepan, until a breadcrumb sizzles gently. (CAUTION: hot oil can be dangerous! Do not leave unattended.)
Fry the noodles for 1-2 minutes, until puffed up and crisp. Remove with a slotted spoon and drain on kitchen paper.
Serve the broth topped with the crispy noodle cake.