For the stock: Add carrot and celery to 2 tbs butter and stir on medium heat for 1 minute. Add mushrooms and saute for 5 minutes. Add beef broth and bring to a boil. Reduce heat and simmer for 20 minutes. Use a blender, or blender stick to blend stock together with the mushrooms. Prepare Roux: Melt 3 tbs butter in shallow saucepan. Add flour, a bit at a time while stirring. Stir for 2 minutes without burning. Add 1½ cups milk, a bit at a time, while stirring. When all milk has been added add this mixture to the soup stock, also stirring continuously. Stir until thickened. Bring to a simmer and cook for additional 5 minutes.