Halve, peel and finely dice the onion. Peel and finely chop the garlic. Give the coconut milk a good shake in its tin before opening. Spoon out 2 tbsp of coconut milk into a bowl and set aside.
Add the chopped onion and garlic to a pan with the coconut milk from the tin and 400ml boiling water. Simmer over a medium heat for 5 mins.
Finely chop the spinach and pile into the pan. Swirl through till just wilted. Immediately take off the heat and blend till smooth. Season with salt and pepper to taste. Dish up and garnish with a swirl of the reserved coconut milk.