Heat oil in a large pot over medium heat. Add onions and celery and cook for about 5 minutes until translucent but not browned. Add garlic and cook for just about 30 seconds until fragrant, taking care not to burn.
Add sweet potato, beets, healthy pinch of kosher salt and enough stock, about 3 cups, to just barely cover the veggies. The veggies don't need to be fully covered as the steam will cook them. Remember, it's always easier to thin the soup out later.
Cut the beets smaller than sweet potatoes, as they take longer to cook. Even consider cutting them into 1/4 inch matchsticks. Bring this to a boil, cover and reduce the heat to maintain a gentle simmer. You will need to simmer this until the veggies are very tender, 30-50 minutes depending how thinly your beets were cut.
When the vegetables are tender, transfer the contents of your pot to a blender. Process until very smooth. Stir in the sour cream. If desired, feel free to add more stock to thin the soup out to desired consistency.
Divide the soup among bowls and serve with garnishes of choice and freshly ground black pepper. Enjoy this hot or cold.