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1
Bring broth to a simmer in a 4-quart heavy saucepan over moderate heat.
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2
Add chicken and simmer very gently, turning once, 6 minutes total.
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3
Remove pan from heat and cover, then let stand until chicken is just cooked through, about 15 minutes.
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4
Transfer chicken to a plate to cool and keep broth warm, partially covered.
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5
Melt butter in a 2- to 3-quart heavy saucepan over low heat, then add flour and cook roux, stirring, 3 minutes.
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6
Add warm broth all at once, whisking, and simmer gently, whisking occasionally, 10 minutes.
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7
Whisk in cream and simmer, whisking occasionally, 5 minutes.
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8
Remove from heat and stir in mustards, capers, salt, pepper, nutmeg, and dill (to taste).
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9
Transfer 1/2 cup sauce to a small bowl and stir in sour cream to make topping.
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10
Shred chicken with your fingers and stir together with remaining sauce in a large bowl.
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11
Preheat oven to 350F.
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12
Cook celery in a large pot of boiling salted water until tender, about 8 minutes.
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13
Transfer with a slotted spoon to a bowl of cold water to stop cooking and return cooking water to a boil.
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14
Drain celery in a colander and add to chicken mixture.
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15
Cook orzo in same boiling water until just tender, then drain in a sieve.
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16
Stir orzo into chicken mixture, then transfer mixture to gratin dish, spreading evenly.
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17
Spoon sour cream topping over top and spread evenly.
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18
Bake, covered with foil, in middle of oven until heated through and sauce is bubbling around edges, 25 to 30 minutes.
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19
Preheat broiler.
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20
Remove foil from dish and broil about 3 inches from heat until top is speckled with brown spots, 3 to 5 minutes.
