Rinse and dry the porcini mushrooms, place in 2 cups of cold water in a bowl, let set for 4 hours or overnight. Place 3 cups of vegetable stock in a large saucepan and turn on medium heat. Meanwhile prepare carrots, onions, and celery, once the stock is at a rolling boil add in the carrots, onions, celery. Then chop the  mushrooms and add to the saucepan. Add the juice that the dried mushrooms soaked in into the saucepan. Rinse the shiitake and the cremini mushrooms and blot dry with a paper towel. Slice the the cremini and shiitake mushrooms and add to the saucepan. Place a tight fitting lid on the saucepan, and reduce the heat to low. Cook this for about an hour or until the vegetables are soft.

Start a roux with the two tablespoons of butter, and the two tablespoons of flour in a small saucepan. Over medium heat cook the flour and butter together until the mixture is bubbly. Take one cup of the vegetable stock from the saucepan full of vegetables and add to the roux, once this mixture has thickened add the roux to the original soup mixture. Stir in chopped parsley and dill. Puree soup with an immersion blender until the soup is smooth. Add heavy cream and whisk until the soup is well blended. Season with salt and pepper to your taste. Garnish with sour cream if you desire.