Bring broth to a simmer in a 4-quart heavy saucepan over moderate heat. Add chicken and simmer very gently, turning once, 6 minutes total. Remove pan from heat and cover, then let stand until chicken is just cooked through, about 15 minutes. Transfer chicken to a plate to cool and keep broth warm, partially covered.
Melt butter in a 2- to 3-quart heavy saucepan over low heat, then add flour and cook roux, stirring, 3 minutes. Add warm broth all at once, whisking, and simmer gently, whisking occasionally, 10 minutes. Whisk in cream and simmer, whisking occasionally, 5 minutes. Remove from heat and stir in mustards, capers, salt, pepper, nutmeg, and dill (to taste).
Transfer 1/2 cup sauce to a small bowl and stir in sour cream to make topping.
Shred chicken with your fingers and stir together with remaining sauce in a large bowl.
Preheat oven to 350°F.
Cook celery in a large pot of boiling salted water until tender, about 8 minutes. Transfer with a slotted spoon to a bowl of cold water to stop cooking and return cooking water to a boil. Drain celery in a colander and add to chicken mixture. Cook orzo in same boiling water until just tender, then drain in a sieve.
Stir orzo into chicken mixture, then transfer mixture to gratin dish, spreading evenly. Spoon sour cream topping over top and spread evenly. Bake, covered with foil, in middle of oven until heated through and sauce is bubbling around edges, 25 to 30 minutes.
Preheat broiler.
Remove foil from dish and broil about 3 inches from heat until top is speckled with brown spots, 3 to 5 minutes.
Cooks' note: Casserole can be assembled (but not baked or broiled) 6 hours ahead and cooled, uncovered, then chilled, covered with foil. Let stand at room temperature 15 minutes before baking.

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Nutrition Facts

Per Serving Per 100 g

Amount Per 1 Serving
Calories	512.02 Kcal (2144 kJ)
Calories from fat	262.48 Kcal
		% Daily Value*
Total Fat	29.16g	45%
Cholesterol	178.77mg	60%
Sodium	2055.88mg	86%
Potassium	1062.78mg	23%
Total Carbs	17.31g	6%
Sugars	3.2g	13%
Dietary Fiber	3.4g	14%
Protein	44.91g	90%
Vitamin C	16.9mg	28%
Vitamin A	0.1mg	4%
Iron	0.4mg	2%
Calcium	105.4mg	11%

Amount Per 100 g
Calories	84.4 Kcal (353 kJ)
Calories from fat	43.27 Kcal
		% Daily Value*
Total Fat	4.81g	45%
Cholesterol	29.47mg	60%
Sodium	338.9mg	86%
Potassium	175.19mg	23%
Total Carbs	2.85g	6%
Sugars	0.53g	13%
Dietary Fiber	0.56g	14%
Protein	7.4g	90%
Vitamin C	2.8mg	28%
Iron	0.1mg	2%
Calcium	17.4mg	11%

* Percent Daily Values are based on a 2000 calorie diet. Your daily values may be higher or lower depending on your calorie needs.
Find out how many calories should you eat.

Tastes

salty
savory
bitter
sweet

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