In a medium pot over high heat, cover celeriac with 1½ cans coconut milk. Add ginger, garlic, lemongrass and salt; cover and bring to a boil, then simmer until celeriac is fork-tender, 25 to 30 minutes.
Remove from heat and cool slightly, 10 minutes. Discard lemongrass and, working in batches, add remaining ½ can coconut milk and purée celeriac-coconut milk mixture until smooth. Again, working in batches, add watercress, honey and juice and purée until smooth.