In a 5 quart saucepan, bring 3 quarts water to a boil. Add chicken and simmer, uncovered, for 20 minutes. At the same time, in a separate saucepan, boil the chunks of potato until barely done, 10-15 minutes. Do not overcook, as they will become too soft and mushy. Drain potatoes in colander. Remove chicken when done and let stand until cool enough to work with. Discard liquid, skin, and bones (or save for broth later). Cut chicken into 3/4″ cubes and place in a large bowl along with potatoes.
In a 3 quart saucepan, bring 2 quarts of water to boil and cook onions until tender, 7-8 minutes. Transfer onions with slotted spoon to a bowl of ice and water, reserving cooking liquid in pan. Cut root ends off onions and peel. Add onions to chicken. Return reserved cooking liquid to a boil and cook carrots and celery until just tender, 4-5 minutes. Drain carrots and celery and add with peas to chicken mixture.
In a pan, cook mushrooms in 1/2 tsp. butter over moderate heat, stirring until softened and golden brown, and all liquid given off is evaporated. Add to chicken mixture.
In a pan, cook shallots with bay leaf, thyme, and tarragon in remaining butter over moderately low heat, stirring, until soft and pale golden. Stir in flour. Cook mixture, stirring, 3 minutes (it will look crumbly). Gradually whisk in wine, broth, nutmeg, salt and pepper to taste. Simmer sauce 5 minutes, whisking occasionally. Whisk in sherry and sour cream. Discard bay leaf.
Preheat oven to 425 degrees. Pour sauce over chicken mixture and gently stir until combined well. Place mixture in baking dish or Dutch oven, and lay crust over top. Bake for approximately 30 minutes, or until crust is golden brown, and filling is hot.