Matzo Ball Soup with Dill-Horseradish Pistou

In a bowl, whisk the eggs with the olive oil, seltzer, salt, white pepper and ginger. Add the matzo meal and stir until moistened. Refrigerate until firm, at least 2 hours. Line 2 baking sheets with wax paper. Scoop the matzo meal mixture into fifty 1-inch balls. Using lightly moistened hands, roll the matzo balls until smooth. Transfer to the baking sheets and refrigerate the matzo balls briefly. In a blender or food processor, pulse the olive oil with the dill, fresh horseradish, garlic, salt and white pepper until the dill is finely chopped and a sauce has formed. Bring a medium saucepan of water to a boil. Add the turnip, celery and carrot and cook the vegetables until they are crisp-tender, about 5 minutes. Drain and refresh under cold water. In a large pot of boiling salted water, simmer the matzo balls over very low heat, covered, until they are plump and cooked through, 25 to 30 minutes. Meanwhile, in another large pot, heat the stock with the vegetables. Using a slotted spoon, transfer the matzo balls to the soup; simmer for 5 minutes. Add the chicken and cook until heated through. Serve the soup in bowls with a dollop of the dill pistou.