Preheat the oven to 350 degrees F (175 degrees C). Spray a 9x13 inch baking dish with cooking spray.
Spread the broccoli evenly in the bottom of the baking dish. Place chicken breasts over the broccoli. In a medium bowl, stir together the condensed soup, sour cream and horseradish. Pour or spread evenly over the chicken and broccoli.
Bake uncovered for 1 hour in the preheated oven. Let stand for a few minutes before serving for the sauce to thicken.
