Peel the top outer layer from the lemon grass and chop the balance very finely.
Place the garlic, lemon grass, oil and chilli into a saucepan and then stir over medium heat for one minute.
Pour in the stock and bring to the boil, then simmer for five minutes.
Wash the bok choy and cut into quarters lengthwise.
Add to the stock and cook along with the udon noodles.
Slice the fish then add until cooked (1-2 minutes).
Cut the lime into quarters or slices, sprinkle with the chilli flakes and serve on top of the broth.