Put the stock on to heat in a small saucepan. Peel and finely chop the garlic and the lemon grass. Finely chop the spring onions. Seed and finely chop the chilli.
Heat the oil in a large, heavy-based saucepan. Add the garlic, lemon grass, spring onions, chilli and curry paste. Cook, stirring, over moderately high heat for 1–2 minutes.
Stir in the stock, coconut milk, galangal and brown sugar. Bring to the boil, reduce heat and simmer gently for 5 minutes. Meanwhile, squeeze enough lime juice to give 1 tablespoon.
Add the lime juice, fish sauce, lime leaves and mushrooms to the pot. Simmer for a further 2–3 minutes.
If necessary, toast the coconut for the garnish in a small frying pan and slice some chilli. When the soup is ready, remove the galangal and pour into a warmed serving bowl. Garnish with lime leaves, shredded coconut, chilli and coriander, and serve immediately.