FOR THE PARSLEY-CHIVE OIL (OPTIONAL)
1/2 cup packed parsley leaves and tender stems
1/4 cup packed chopped chives
1/3 cup olive or grapeseed oil
FOR THE SOUP
2 tablespoons unsalted butter, or more as needed
2 tablespoons extra-virgin olive oil, or more as needed
2 medium leeks (thoroughly cleaned), white and light-green parts, coarsely chopped (2 cups)
2 medium cloves garlic, very thinly sliced
Sea salt
2 large or 4 small celery root (celeriac; about 4 pounds total), peeled and cut into 3/4-inch chunks (8 cups)
1 large Granny Smith or other tart apple, peeled, cored and cut into 1-inch chunks
3 cups homemade or no-salt-added chicken broth, or more as needed
2 cups water, or more as needed
1 cup apple juice, preferably unsweetened (may substitute apple cider)
1 to 1 1/2 teaspoons white pepper, preferably freshly ground
Scant 1/8 teaspoon chili powder
Roasted mushroom chips, for garnish (see NOTE)