Heat a deep frying pan over a medium heat. Add the curry paste and cook for 1 minute. Add the coconut milk, stock, fish sauce and sugar. Bring to the boil, then reduce the heat and simmer for 2 minutes.
Add the prawns and cook for a further 2 minutes. Stir in the vegetable stir-fry, spring onions, noodles and lime zest. Simmer for 2-3 minutes, until the prawns are cooked through and piping hot. Stir in the lime juice and chopped coriander.
To serve, divide the soup among bowls, then add the spinach and garnish with the extra coriander.