Preheat your oven to 425°F (220°C). Prepare a rimmed baking sheet.

Toss chopped celery and Yukon Gold potatoes with olive oil, salt, and pepper in a mixing bowl. Spread on the baking sheet.

Roast vegetables in the oven for 25-30 minutes until golden-brown and tender, stirring halfway through.

In a large saucepan, heat the remaining olive oil over medium heat. Sauté onion for 5 minutes until translucent. Add garlic and thyme; stir for 1 minute.

Combine roasted vegetables with onion and garlic mixture. Add vegetable broth and bring to a gentle boil. Reduce heat and simmer for 10-15 minutes.

Blend the soup until smooth using an immersion blender. Add heavy cream if desired and heat through.

Ladle into bowls and garnish with chives or parsley before serving.