Place the prawn shells and heads, chilli, tomato and stock into a saucepan. Bring to the boil, then reduce the heat until the mixture is simmering and simmer for 4-5 minutes.
Strain the mixture through a sieve, discarding the shells and tomato pieces. Reserve the liquid.
Chop the cooked noodles and place them into a soup bowl. Pour over the fish broth.
To serve, scatter over the chopped herbs.
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