Cook noodles in a large saucepan of boiling water until tender and then drain. Rinse under cold water.
Heat oil in a large saucepan over medium heat. Add the garlic, lemon grass and ginger and gently cook until fragrant. Add the curry paste and cook 30 seconds. Add the stock, soy sauce and sugar and bring to a boil. Reduce the heat and simmer 20 minutes.
Stir in coconut milk, prawns, mushrooms, spinach, lime juice, and coriander. Cook for 5 minutes or until prawns just change colour.
To serve, place rice noodles into serving bowls and ladle over soup. Garnish each bowl with spring onion.