01 -
Preheat oven to 220°C. Arrange the celery and potatoes on a baking sheet. Drizzle with 2 tablespoons olive oil, season with salt and pepper.
02 -
Roast vegetables for 25-30 minutes, turning halfway through cooking, until golden brown and tender.
03 -
In a large pot, heat 1 tablespoon olive oil over medium heat. Add the onion and cook until softened, about 5 minutes. Add the garlic and thyme, cooking for 1 more minute.
04 -
Add the roasted celery and potatoes to the pot and pour in the vegetable broth. Bring to a boil, then reduce to a simmer for 10-15 minutes.
05 -
Use an immersion blender (or carefully transfer to a regular blender in batches) to blend the soup until smooth and creamy.
06 -
Stir in the heavy cream if using, and season to taste with additional salt and pepper.
07 -
Serve hot, garnished with chopped chives or parsley.