Mustard Chicken and Orzo Casserole with Dill and Capers

Preparation

1. Step 1

   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.

   Step 2

   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).

   Step 3

   Transfer 1/2 cup sauce to a small bowl and stir in sour cream to make topping.

   Step 4

   Shred chicken with your fingers and stir together with remaining sauce in a large bowl.

   Step 5

   Preheat oven to 350°F.

   Step 6

   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.

   Step 7

   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.

   Step 8

   Preheat broiler.

   Step 9

   Remove foil from dish and broil about 3 inches from heat until top is speckled with brown spots, 3 to 5 minutes.