Make the Stock: Combine all the ingredients in a large pot and bring to a boil. Lower the heat and simmer for 45 minutes. Strain the stock, return to the pot, and bring to a boil. Reduce the stock to 2 1/2 cups. (The stock can be made up to 3 days in advance and refrigerated, covered.)
Make the Soup: Combine all of the ingredients in a non-reactive pot, season with salt, to taste, and bring to a boil. Lower the heat and simmer until the celeriac is tender.
Strain the soup, reserving the celeriac and liquid separately. Working in small batches, puree the celeriac with enough liquid to let the blender blades spin, and transfer to a bowl. Whisk enough liquid into the puree to reach a soup-like consistency. Strain the soup through a fine-mesh sieve. . (The soup can be made up to 3 days in advance and refrigerated, covered.)
To make the Glazed Celeriac: Combine all the ingredients in a large skillet over medium heat and cook, stirring occasionally, until the water has evaporated and the celeriac is glazed. Set aside.
To make the Curried Apple and Celery: Preheat the oven to 300 degrees F. Combine the honey, butter, curry powder, and salt in a medium oven-proof skillet and cook, until fragrant and bubbling, stirring occasionally over medium heat. Add the apples and baste them with the curry mixture, using a kitchen spoon. Transfer the skillet to the oven and bake, basting every 15 minutes, until the apples are tender and glazed, about 1 hour. Let the apples and cool. Core the apples and cut into slices about 1/16 inch thick. Set aside. (The apples can be made up a day in advance and refrigerated, covered.)
Meanwhile, bring a small saucepan of salted water to a boil. Using a peeler, peel the celery, removing the strings. Thinly slice the celery on the diagonal, add to the water, and cook until crisp-tender, about 30 seconds. Using a slotted spoon, transfer the celery to a bowl of ice-water, to stop the cooking. Drain and set aside.
When ready to serve, arrange the glazed celeriac, curried apple, and celery in the bottom of 8 soup plates. Meanwhile, bring the soup to a boil in a teapot. Place the soup plates in front of your eagerly awaiting guests, and dramatically pour the soup from the teapot into each plate.
Recipe courtesy of Chef Grant Achatz, Trio Restaurant, Chicago, IL