Preheat the oven to 425 degrees F (220 degrees C).

Combine parsnips, carrots, turnips, butternut squash, celery root, and sweet potato in a large roasting pan. Drizzle with olive oil; season with 1 teaspoon salt and 1/2 teaspoon black pepper. Toss vegetables to evenly distribute seasonings.

Roast in the preheated oven until vegetables are easily pierced with a fork, 30 to 45 minutes, stirring every 15 minutes.

Meanwhile, melt butter in a large pot or Dutch oven over medium heat. Stir in diced celery and onion; cook and stir until onion softened and turned translucent, about 5 minutes. Add vegetable broth; bring to a simmer, uncovered. Stir in roasted vegetables; continue simmering for 10 minutes. Purée soup with an immersion blender until smooth. Stir in half-and-half; season with salt and black pepper. Add more vegetable broth if soup seems too thick.