Preheat the oven to 375 degrees F. Spray a 13 by 9-inch baking dish with nonstick cooking spray.
In a large skillet, melt the butter over medium-heat. Add the mushrooms and saute until browned, 7 to 8 minutes. To the skillet, add the sour cream, chicken soup, almonds, poppy seeds, and chicken. Pour the chicken mixture into the prepared baking dish.
In a small bowl, combine the melted butter and crackers; sprinkle over the chicken mixture. Cover with aluminum foil and bake about 15 minutes. Uncover and bake until bubbly, about 10 minutes longer. Serve over rice, if desired.