Place the porcini mushrooms in a bowl and cover with the water. Set aside to rehydrate while you dice the mushrooms and garlic.
Heat the oil in a medium pot over medium-low heat. Add the onion and gently fry, stirring from time to time, while you clean and slice the mushrooms.
When the onion is transparent, add the garlic and fry for another 30 seconds to a minute until soft and fragrant.
Add the sliced mushrooms and thyme and raise the heat to medium-high. Allow the mushrooms to cook, stirring frequently to avoid burning on the bottom, until they begin to release their liquid and are soft - about 5 minutes.
Remove the porcini mushrooms and give them a squeeze to get out some of the water. Chop them into smaller pieces if necessary.
Add the vegetable stock, salt and mushroom soaking liquid to the pot. Be careful when adding the mushroom stock because there will be some grit and sand on the bottom of the bowl, don’t add that.
Bring to a boil and simmer for 10 minutes. Remove the pot from the heat and add the porcini mushrooms and coconut milk.
Carefully transfer 3 - 4 ladlefuls (about 1 cup / 250 ml) to a blender and blend until creamy. Return to the pot, stir it well and serve.