For the broth, simmer the fish trimmings, onion and just-boiled water in a saucepan for 10-15 minutes. Sieve the liquid into another saucepan and discard the trimmings and onion.
For the fish, place a piece of greased silicone paper in a bamboo steamer set over a pan of boiling water. Place the fish on top, skin-side down. Season well with salt and freshly ground black pepper. Scatter the remaining ingredients over the top.
Cover the pan with a lid and steam the fish for 5-7 minutes, or until it is cooked through (you will know the fish is cooked when the flesh is opaque).
To serve, heat up the broth with the cooked noodles and pour it into a bowl. Place the steamed fish into the broth.