Place a large pot over medium heat and add the canola oil and butter. Once the butter is melted, stir in the leeks, onion and celery. Turn the heat to medium low and cook for 10 min until the vegetables are soft and translucent.
Add the potatoes, chicken stock and fresh thyme sprigs. The stock should just cover the vegetables by about 1-inch, add more stock as needed. Cover the soup and bring to a simmer, cook for 15-20 minutes or until the potatoes are tender.
Remove the thyme sprigs. Carefully transfer the contents of the soup pot to a blender (or use an immersion blender). If the blender is small, blend the soup in two batches. Untwist the funnel cap from the top of the blender to allow the steam to escape, but dont take it off. Use a kitchen towel to loosely cover the cap. Blend until the soup is thick and creamy.
Return the blended soup to the pot and stir in the buttermilk and dill. Heat over a very low burner. Don’t bring the soup to a boil again as the buttermilk might curdle. Serve.
