Trim and roughly slice the onions, including the green.
Heat the oil in a heavy-based pan, stir in the onion and cook until soft.
Stir in the Thai curry paste, cooking for a further 20 seconds.
Add the coconut cream, fish sauce, 2 strips lime leaves and the lime juice.
If you are using lemongrass, bash with a rolling pin and add to the pot. Simmer for 10 minutes.
Dissolve the stock cube in 250ml boiling water, add the stock and return to the simmer. Add the spinach.
Once melted, add the prawns.
Chop the coriander stalks and stir into the pot.
Warm through and serve with a lime wedge to squeeze into the soup.