Heat a wok over low heat, add the oil and swirl. Cook the paste for 3 to 5 minutes, stirring constantly. Pour in 800 to 900 ml of stock and bring to the boil, then reduce the heat and simmer for 15 minutes, or until reduced slightly.

Add the coconut milk, lime leaves, lime juice, fish sauce and sugar and simmer for 5 minutes. Add the prawns and simmer for 2 minutes, or until pink and cooked. Do not boil or cover.

Meanwhile, soak the vermicelli in boiling water for 6 to 7 minutes, or until soft. Drain and divide among serving bowls along with most of the sprouts. Ladle on the hot soup, then top with the tofu, mint, coriander and the remaining sprouts. Serve with lime wedges.