Poppy Seed Chicken Casserole

1 Tablespoon butter
8 oz. mushrooms, sliced
3 cups chicken, cooked and cubed
1 can cream of celery soup
1 cup sour cream
2 ounces pimiento, drained

Topping:
1/2 cup butter
1 1/3 cups Ritz crackers -- crushed
2 teaspoons poppy seeds

Melt margarine. Saute mushrooms until tender. Stir in cooked chicken, soup, sour cream, and pimento. Mix well. Spoon into a buttered 2-quart casserole.
In small bowl, combine crushed crackers (almost one sleeve of Ritz crackers), 1/2 cup butter and 2 tsps poppy seeds. Sprinkle over chicken.
Bake at 350 degrees for 20 minutes.