Heat the oil in a large pot over medium heat. Add parsnip and carrots first and let roast on high heat for about 10 minutes. Now add onion and garlic, lower the heat and let cook for 10 minutes more.
Add the stock and bring to a boil. Reduce heat to low and simmer for 20 minutes.
Puree the soup with a blender and add cream, if desired. Season with salt and pepper to taste. Serve hot.