Step 1

Heat oil in a large pan over medium heat. Cook lemongrass, galangal and garlic for 30 seconds or until fragrant. Add kaffir lime and stock, increase heat to medium-high and bring to the boil

Step 2

Add chillies and chilli paste, and simmer for 5 minutes or until fragrant. Add tomatoes and mushrooms, and simmer for 5 minutes or until tender.

Step 3

Add prawns and simmer for 2 minutes or until cooked through, then stir in lime juice, fish sauce and coconut sugar.

Step 4

Divide soup among bowls to serve.