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1
Make filling: In a 5-quart saucepan bring 3 quarts water to a boil.
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2
Add chicken and simmer, uncovered, 20 minutes.
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3
Remove pan from heat and let stand 30 minutes.
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4
Transfer chicken to a bowl, discarding cooking liquid, and cool.
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5
Discard skin and bones.
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6
Cut chicken into 3/4-inch cubes and return to bowl.
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7
In a 3-quart saucepan bring 2 quarts water to a boil and cook onions until tender, 7 to 8 minutes.
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8
Transfer onions with a slotted spoon to a bowl of ice and cold water, reserving cooking liquid in pan.
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9
Cut root ends off onions and peel.
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10
Add onions to chicken.
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11
Return reserved cooking liquid to a boil and cook carrots and celery until just tender, 4 to 5 minutes.
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12
Drain carrots and celery and add with peas to chicken mixture.
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13
In pan cook mushrooms in 1/2 tablespoon butter over moderate heat, stirring, until softened and golden brown and all liquid they give off is evaporated and add to chicken mixture.
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14
In pan cook shallots with bay leaf, thyme, and tarragon in remaining 1 1/2 tablespoons butter over moderately low heat, stirring, until soft and pale golden and stir in flour.
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15
Cook mixture, stirring, 3 minutes (it will look crumbly) and gradually whisk in wine, broth, nutmeg, and salt and pepper to taste.
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16
Simmer sauce 5 minutes, whisking occasionally, and whisk in Sherry.
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17
Discard bay leaf.
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18
Pour sauce over chicken mixture and stir until combined well.
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19
Filling may be made 1 day ahead, cooled, uncovered, and chilled, covered.
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20
Preheat oven to 425 F.
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21
Make mashed potatoes:
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22
Peel potatoes and cut into 1-inch pieces.
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23
In a saucepan cover potatoes with water by 1-inch simmer until very tender, 10 to 15 minutes.
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24
Reserve about 1/3 cup cooking liquid and drain potatoes.
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25
In batches, force potatoes through a ricer or food mill fitted with medium disk into a bowl and stir in sour cream, chives, salt and pepper to taste, and enough reserved cooking liquid to reach a fluffy consistency.
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26
Transfer potatoes to a pastry bag fitted with a 1/2-inch fluted tip.
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27
Spread filling in a 1 1/2- to 2-quart gratin dish or other shallow baking dish.
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28
Pipe mashed potatoes decoratively in mounds on filling and bake in middle of oven until filling is bubbling and potatoes are golden on edges, about 25 minutes.
