01 - Set the oven to 425°F and prepare for roasting the vegetables.
02 - Place parsnip, carrots, sweet potato, onion, and unpeeled garlic cloves on a large baking sheet. Drizzle with 2 tablespoons olive oil, add dried thyme, rosemary, salt, and pepper, then toss to coat evenly.
03 - Cook in the oven for 25 to 30 minutes, turning halfway through until vegetables are golden and tender.
04 - While roasting, heat remaining 1 tablespoon olive oil in a large pot over medium heat. Add chopped celery and sliced leek, then sauté for about 5 minutes until softened.
05 - Squeeze the roasted garlic from skins into the pot. Add the other roasted vegetables and stir to combine.
06 - Pour in vegetable broth and add the bay leaf. Bring to a boil, then reduce heat and simmer for 10 to 15 minutes.
07 - Remove the bay leaf, then blend the soup using an immersion blender or transfer in batches to a blender until smooth. Adjust consistency with additional broth or water if necessary.
08 - Stir in optional apple cider vinegar, taste, and adjust seasoning with salt and pepper as needed.
09 - Ladle hot soup into bowls, garnish with chopped fresh parsley and a spoonful of crème fraîche or yogurt if desired.