Preheat the oven to 350 degrees F.
Arrange the birds in a casserole dish. In a medium saucepan, melt the butter and then saute the onion, mushrooms, and celery. Add the flour and stir well to blend. Slowly add the milk to make a cream sauce. Season with salt and pepper, to taste, and stir in the gravy seasoning sauce.
Pour the mixture over the birds. Cover the dish with foil and bake for approximately 20 minutes. Remove some of the cooking liquid and combine with the sour cream. Bake uncovered for approximately 20 minutes more. Serve with the sour cream sauce.