Preheat your oven to 425°F (220°C).

Toss chopped celery and Yukon Gold potatoes with 2 tablespoons of olive oil, salt, and black pepper, then spread on a baking sheet. Roast for 25–30 minutes, flipping halfway.

Heat 1 tablespoon of olive oil in a large pot over medium heat. Add diced yellow onion and sauté for 5 minutes. Stir in minced garlic and dried thyme, cooking for another minute.

Add roasted vegetables to the pot, pour in vegetable broth to cover, and bring to a gentle boil. Reduce heat to low and simmer for 10–15 minutes.

Remove from heat and carefully use an immersion blender to purée the soup until smooth.

Return pot to low heat and stir in heavy cream if using. Adjust seasoning as desired.

Ladle soup into bowls and garnish with chopped chives or parsley.