"In a large saucepan, combine mushrooms, celery, 1/2 stick butter and pepper. Saute over medium heat for 15 minutes or until vegetables have cooked down and released their juices. Remove from heat and set aside. Do not discard liquid. While mushrooms and celery are cooking, in a large saucepan, melt remaining 2 1/2 tablespoons butter over medium heat and stir in flour. Cook for 1 minute, stirring constantly. Add water in 1/2 cup increments, stirring between each addition to be sure no lumps form. Add beef base and chicken base and bring mixture to a slow boil. Reduce heat and simmer 15 minutes. Soup will thicken. Stir in milk, half-and-half and reserved mushrooms and celery, with their cooking liquid. Season to taste with salt and pepper. Serve hot."