Heat the oil in a large pan on medium. Add the onions, chopped parsley and coriander stalks. Lower heat and cook until soft without browning, about 10 minutes.
Add the garlic and potato and stir.
Pour in the stock, parsley leaves and half the coriander leaves. Simmer for 10-15 minutes, or until potato are cooked. Meanwhile chop remaining coriander leaves and set aside.
When the potatoes are cooked, remove from heat and blend it to a smooth purée with a hand blender.
Pour the soup back into the pan, and season with salt, pepper, lemon juice and curry paste to taste. Add and stir well the remaining coriander leaves and milk. Serve in soup bowl warm or cold.