1.Using a mortar and pestle, pound garlic, chilli, and coriander roots and stems to a paste.
2.Heat oil in a large saucepan on medium-high. Add paste and cook, stirring, for 1 minute, until fragrant. Add stock, water, soy sauce, vinegar and sugar. Bring to boil. Reduce heat to medium and simmer for 5 minutes, until reduced slightly. Add noodles and cook for 5 minutes, until just tender. Add prawns and cook for 2 minutes, until prawns are opaque.
3.Divide soup between serving bowls. Top with bean sprouts and coriander leaves and serve.