Crunchy Chicken, Mushroom, and Green Pea Casserole

6 tablespoons (3/4 stick) butter
1 pound boneless, skinless chicken breasts, cubed
1 medium-size leek (white part and some of the green), washed well, patted dry, and cut into pieces
1 pound fresh mushrooms, sliced
1 cup frozen green peas
2 cups sour cream
2 teaspoons Dijon mustard
Salt and pepper to taste
1/2 cup all-purpose flour
1/2 cup rolled oats
1 cup soft bread crumbs
1/4 cup freshly grated Parmesan cheese
1 teaspoon dried tarragon, crumbled

1. Preheat the oven to 350 degrees.

2. In a flameproof 1 1/2-quart casserole, melt 2 tablespoons of the butter over moderate heat, then add the chicken and brown till just olden on all sides, 5 to 6 minutes. Add the leek and stir till softened, about 5 minutes. Add the mushrooms and peas and stir till the peas have thawed, about 3 minutes. Add the sour cream and mustard, season with salt and pepper, and remove from heat.

3. In a medium-size mixing bowl, combine the flour, oats, and the remaining 1/4 cup butter and work with a work till the mixture is mealy. Add the bread crumbs, cheese, and tarragon, stir till well blended, spoon evenly over top of the chicken, and bake till golden brown, 35-40 minutes.