Roast: Toss celeriac with oil, salt, and thyme. Roast at 400°F (200°C) for 25–30 mins until tender. Toast hazelnuts separately for 10 mins.
Sauté: In a pot, cook onion until soft. Add garlic and nutmeg; stir for 1 minute.
Blend: Add celeriac, ¾ cup hazelnuts, and stock. Simmer, then blend until smooth.
Finish: Stir in cream. Season with salt and pepper.
Serve: Garnish with chopped hazelnuts, thyme, and a drizzle of cream.