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1
Squeeze the juice of 1 to 2 lemons into a large bowl of cold water and keep it close by as you work with the celery root. To clean the celeriac, trim the leaves and the stalks (set them aside for another use, if you wish). Peel the rough, gnarly skin by cutting a slice off the top and the bottom (the way you would peel a melon or pineapple). Then, carefully slide your knife down the sides, taking the peel off as you go. Cut the celery root into 1/2-inch cubes and immerse the slices into the lemon water as you work.
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2
Peel and dice the potatoes. Because the potatoes require slightly more time to cook than the celery root, be sure to cut the potatoes into roughly the same size or smaller. Chop the celery stalks into thin slices (and if they have leaves, trim and save the leaves for the salad below).
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3
Heat the oil in a soup pot over medium-high heat. Drain the celeriac (discard the lemon water) and then add the celery, celeriac, potatoes, shallots, and parsley to the soup pot. Cook, stirring from time to time, until the vegetables develop some color -- about 8 to 10 minutes. Add the garlic, wine, and 1 teaspoon sea salt, and cook for a few more minutes until the wine has reduced. Add 6 cups of stock (or water) and bring to a boil. Lower the heat and simmer, partially covered, for about 30 minutes or until the vegetables are tender. Take the soup off the heat and allow it to cool for a few minutes before transferring to a blender.
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4
Blend or pulse the soup very briefly until smooth but leave a good amount of texture. Potatoes also tend to get gummy if blended too long.
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5
Prepare the vinaigrette for the salad: In a small bowl, whisk together 1 tablespoon red wine vinegar, 1/2 teaspoon Dijon mustard, and 2 tablespoons walnut oil. Season with sea salt and black pepper.
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6
Trim celery leaves from two bunches of celery. Rinse and dry the leaves and then set them aside.
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7
Toast the walnuts lightly.
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8
Heat 1 tablespoon olive oil over medium-high heat in a large skillet. When the olive oil becomes fragrant, add the mushrooms and saute for a few minutes, stirring frequently, until they become brown and begin to release some water. Add 1/4 teaspoon sea salt and some freshly ground black pepper and cook for a few more minutes, stirring often, until the mushrooms are fully cooked through. Toss the mushrooms, walnuts, parsley, and celery leaves with the Dijon mustard vinaigrette.
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9
Serve the soup warm and top each bowl with two heaping tablespoons of the salad. Both the soup and salad will keep in the fridge overnight. The soup may be safely stored in the fridge for up to three days.
