2.Place prawn shells, ginger, peppercorns, garlic, half the chilli, lemongrass and water in a large saucepan or stockpot. Bring to the boil. Reduce heat; simmer, uncovered, 20 minutes. Strain stock; return liquid to clean saucepan.
3.Add prawns to stock; simmer, covered, about 3 minutes, or until prawns are changed in colour.
4.Meanwhile, pour boiling water over rice noodles in bowl; drain well.
5.Add lemon juice to stock; gradually add fish sauce to taste. Divide prawns and noodles evenly among 6 serving bowls. Top with stock, green onions, herbs and remaining chilli.
This recipe is best made just before serving. It is not suitable to microwave.