How to Make It
In a large pot, combine the kombu with 10 cups of water and bring just to a simmer. Cover and let stand off the heat for 10 minutes. Discard the kombu. Add the shiitake to the pot along with the bay leaf, thyme and peppercorns and bring to a boil. Cover and let stand off the heat for 10 minutes. Strain the dashi (broth) into a heatproof bowl; you should have 8 cups. Discard the solids and wipe out the pot.
In the same pot, heat 1 tablespoon of the oil. Add the onion, carrot, celery and garlic and cook over low heat until softened, about 8 minutes. Add the lentils and the 8 cups of dashi and bring to a boil. Cover partially and simmer over moderately low heat until the lentils are tender, about 1 hour.
Working in batches, puree the soup in a blender or food processor with the miso until smooth. Return the soup to the pot; add a few tablespoons of water if it is very thick. Season with salt and pepper and keep warm.
In a medium skillet, melt the butter in the remaining 1 tablespoon of grapeseed oil. Add the celery root, season with salt and pepper and cook over moderate heat, stirring occasionally, until crisp-tender and lightly browned, about 7 minutes. Transfer to a plate and keep warm.
In the same skillet, cook the bacon over moderate heat until browned and crispy, 5 to 6 minutes. Strain the fat into a heatproof bowl and reserve 3 tablespoons. Drain the bacon on paper towels.
large heatproof bowl, whisk the egg yolks with 11/2 teaspoons of warm water. Set the bowl over a pan of simmering water (do not let the bowl touch the water) and whisk the eggs constantly until thick and ribbony, about 5 minutes. Whisk the 3 tablespoons of reserved bacon fat into the sabayon in a thin stream and cook until thick and billowy, about 1 minute. Gradually whisk in the cider vinegar and soy sauce and whisk for about 1 minute longer.
Serve the soup in shallow bowls and garnish with the celery root, sabayon and crispy bacon.