Preheat the oven to 200 C.
Toss the celeriac and parsnip with the olive oil. Arrange the vegetables in a single layer on a foil-covered baking sheet. Roast them in the preheated oven for 35 to 45 minutes, stirring once, until they are tender and have turned golden brown. Transfer the caramelized vegetables to a large stockpot.
Bring the vegetables, stock, bay leaf, salt, and black pepper to a boil over medium-high heat.
Reduce the heat, and simmer the soup for 15 minutes.
Discard the bay leaf and process the soup in a blender or food processor until it is smooth.
Serve the soup hot, drizzled with pistou as a garnish.
