Ingredients

  • 1 tablespoon olive oil
  • 1 medium yellow onion, finely chopped
  • Kosher salt
  • Freshly ground black pepper
  • 1 teaspoon ground mustard
  • 4 cups (1 quart) low-sodium vegetable broth
  • 3 medium turnips (about 1 pound), peeled and medium dice
  • 2 medium parsnips (about 8 ounces), peeled and medium dice
  • 2 bay leaves
  • 1 medium celery root (about 12 ounces), peeled and medium dice
  • 1 cup water
  • 4 ounces watercress, stems trimmed, washed, and dried
  • 5 medium garlic cloves, coarsely chopped
  • 1 tablespoon finely grated Parmesan cheese
  • 1/4 cup olive oil
  • Kosher salt
  • Freshly ground black pepper

Method

  • Heat the oil in a medium saucepan over medium heat until shimmering.
  • Add the onion, season with salt and pepper, and cook, stirring occasionally, until softened and just beginning to turn golden brown, about 5 minutes.
  • Add the mustard, stir to coat the onion, and cook until fragrant, about 1 minute.
  • Add the remaining ingredients, season generously with salt and pepper, and stir to combine.
  • Bring to a boil, then reduce the heat to medium low, cover, and simmer until the vegetables are tender, about 15 minutes.
  • Meanwhile, combine the watercress, garlic, and Parmesan in a blender.
  • Remove the small cap from the blender top, cover the hole with a kitchen towel, and turn the blender on.
  • Remove the towel and add the oil in a slow, steady stream until evenly incorporated.
  • Stop and scrape down the sides of the blender, then pulse until smooth.
  • Taste and season with salt and pepper as needed.
  • Remove the soup from the heat and discard the bay leaves.
  • Taste and season with additional salt and pepper as needed.
  • Ladle the soup into serving bowls and top each bowl with a heaping teaspoon of the pistou.