Add 2 tbs olive oil to a soup pot along with the carrots, onion, and celery. Cook until softened and translucent, about 5 minutes, then add the garlic and cook until fragrant (about 1 minute).
Add the potatoes, stock, black pepper, and ground mustard. Bring to a boil, then lower slightly and cook until the potatoes are tender, about 20 minutes.
Add the pickles, pickle juice, bay leaves, and dill. Stir and let simmer for about 30 minutes.
In the meantime, whisk together the sour cream and flour and set aside.
Once the soup is almost done, take ¼–½ cup of the simmering broth and add it to the sour cream mixture. Whisk together until it becomes creamy and then pour it into the soup. Mix together, adjust seasoning as needed, and serve with another dollop of sour cream and more fresh dill.