Preheat your oven to 425°F (220°C). Line a baking sheet with parchment paper.

Spread the chopped celery and Yukon Gold potatoes on the baking sheet. Drizzle with 2 tablespoons of olive oil, then season with salt and black pepper. Roast for 25-30 minutes until golden brown and tender.

Heat 1 tablespoon of olive oil in a large pot over medium heat. Sauté the chopped yellow onion for about 5 minutes until soft. Add minced garlic and dried thyme, cooking for an additional minute.

Add the roasted vegetables to the pot, pour in 4 cups of vegetable broth, and bring to a boil. Reduce heat to a simmer and cook for 10-15 minutes.

Use an immersion blender to puree the soup until smooth. Alternatively, blend in batches in a traditional blender.

Stir in ½ cup of heavy cream if desired. Adjust seasoning with additional salt and pepper before serving hot, garnished with chopped chives or parsley.