Directions

1. Rinse dried mushrooms. Place mushrooms in 2 cups of cold water, and soak for at least 4 hours or overnight.
2. In a large pot, bring stock to a simmer. Add celery, onions, and carrots. Strain dried mushrooms, reserving the soaking liquid. Add strained soaking liquid to soup. Chop hydrated mushrooms into 1/4-inch pieces slightly larger than diced vegetables, and add to soup. Add sliced white button mushrooms.
3. Cover, and cook soup until vegetables are tender, about an hour. Bring soup to a boil. Stirring constantly, add orzo. Reduce the heat to a gentle boil, and, stirring occasionally to prevent pasta from sticking, cook until orzo is cooked through, another 6 to 8 minutes.
4. Meanwhile, make a roux: Melt butter in a small saucepan over medium heat. Add flour, and cook, stirring constantly until smooth, 3 to 5 minutes. Remove 1 cup broth from soup,and add roux, whisking constantly until slightly thickened and free of lumps. Stir thickened liquid into soup. Add chopped parsley and dill.
5. Finish soup with sour cream: Add 1/4 cup thickened soup to sour cream. Whisk until smooth. Add sour cream to soup, whisking constantly until it is well incorporated, about 3 minutes. Season to taste with salt and pepper.