Peel and chop the sunchokes into coins. In a soup pot, heat the grapeseed oil over medium heat. Add the leeks, the garlic, the thyme, and generous pinches of salt and pepper. Sauté until very soft, but not browned, about 15 minutes. Lower the heat if necessary.

Add the sunchokes and the vegetable broth. Bring to a boil, reduce the heat, and simmer until the sunchokes are tender, about 15 minutes. Add the coconut milk, heat through, and turn off the heat. Discard thyme sprig. Mix with an immersion blender or blend in a blender. Serve warm in soup bowls, garnished with garlicky croutons and crispy sage.