Preheat oven to 400 degrees.
Peel and chop potatoes and parsnips into 1-inch pieces.
Place potatoes and parsnips in a large bowl and add in the oil. Mix until well-coated.
Spread onto a cookie sheet pan and roast for 30-40 minutes, or until light golden brown in color and pieces are soft.
While vegetables are roasting, prepare the leek by cutting off and discarding the root end and the thick dark green part of the leek.
Cut the leek in half lengthwise and rinse thoroughly under cold water. Make sure to pull the layers apart and rinse well. Leek may have dirt in between the layers.
Once the leek is washed well, chop into small pieces.
On low heat, melt butter in a dutch oven or medium size soup pot. Add leeks and thyme and sauté on low until leeks are well softened, stirring occasionally.
When vegetables are done roasting, put half of them in a blender, along with half the leeks and half the vegetable broth. Puree until it becomes smooth and creamy.
Pour the puree back into the dutch oven.
Put in the remaining broth and roasted vegetables.
Stir well, and then add the buttermilk and half & half creamer. Do not bring soup to a boil, buttermilk will curdle.
Taste for seasoning and add pepper and salt to taste. Warm soup through and serve.