Using a pestle and mortar or small blender,pound or mix garlic, coriander stalks and leaves and peppercorns to a paste. In a wok, heat oil, add paste and cook, stirring, for 1 minute. Stir in stock, mushrooms and fish sauce. Simmer for 5 minutes.
Add chicken, lower heat so liquid barely moves and cook gently for 5 minutes. Scatter spring onions (scallions) over surface and garnish with coriander sprigs.