Preheat your oven to 425°F (220°C).

On a large baking sheet, toss the chopped celery and cubed Yukon Gold potatoes with olive oil, salt, and black pepper. Roast for 25-30 minutes, turning halfway through.

Heat a tablespoon of olive oil in a large pot over medium heat. Add the diced yellow onion and sauté for about 5 minutes until softened.

Add the minced garlic and dried thyme. Cook for an additional minute while stirring.

Carefully add the roasted celery and potatoes to the pot. Pour in the vegetable broth, bring to a boil, then reduce to a simmer for 10-15 minutes.

Blend the soup until smooth with an immersion blender or in batches with a countertop blender.

Stir in heavy cream if desired and adjust seasonings with salt and pepper.

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