In a large saucepan or Dutch oven, heat the canola oil over medium-high heat.
Add the sliced leeks and cook, stirring frequently, for about 3 minutes or until they are soft and tender.
Stir in the vegetable broth, pumpkin purée, dried thyme (if using), and black pepper.
Bring the mixture to a boil, then reduce the heat to low, cover, and let it simmer for 20 minutes to allow the flavors to meld.
Remove the saucepan from the heat and let it cool slightly. If you used dried thyme, stir in the fresh snipped thyme now.
Carefully transfer half of the pumpkin mixture to a blender.
Cover and blend until it's almost completely smooth.
Pour the blended soup into a separate bowl and repeat with the remaining half of the pumpkin mixture.
Return all the blended soup to the saucepan over low heat.
Stir in half of the vegan sour cream (1/2 cup / 120 g / 4.2 oz) and heat through gently, being careful not to let it boil.
Ladle the soup into four bowls. Swirl a dollop of the remaining vegan sour cream into each serving. If desired, garnish with fresh thyme sprigs before serving.
