1To prepare matzo balls: Pulse mushrooms in a food processor until finely chopped. Heat 2 teaspoons oil in a medium skillet over medium heat. Add the mushrooms and cook, stirring frequently, until browned, 5 to 7 minutes. Add garlic and cook, stirring, for 1 minute. Transfer to a bowl to cool for 10 minutes.
2Whisk the 4 egg yolks into the mushrooms; whisk in seltzer and the remaining ¼ cup oil. Add matzo meal, herbs, 1 teaspoon salt, baking powder, pepper, nutmeg and ginger; stir until evenly moistened.
3Using an electric mixer, beat the 4 egg whites until very fluffy and starting to hold their shape. In 3 additions, fold the whites into the matzo mixture. Refrigerate for 1 hour.
4Put a large pot of water on to boil.
5Using wet hands, scoop walnut-size pieces of dough and lightly roll to make 24 matzo balls. When they're all rolled, add to the boiling water. Reduce heat to a lively simmer; cook, gently stirring from time to time, until the matzo balls are light and fluffy, about 45 minutes.
6To prepare soup: Heat oil in a large pot over medium heat. Add shallot and cook, stirring, until soft, about 3 minutes. Add broth, carrots, celery root and salt; bring to a simmer. Cook until tender, 15 to 20 minutes. Remove from heat. Add the cooked matzo balls to the soup. Serve garnished with pepper and herbs.
Find celery root (a.k.a. celeriac), a celery-flavored root vegetable with a cooked texture similar to potatoes, in the refrigerated produce section near other root vegetables. To peel it, trim off the root and stem end, then remove the thick skin with a knife or use a vegetable peeler and peel around the root several times to remove the fibrous skin. Try it in soups, roasted or mashed like potatoes.