In a large mixing bowl, combine shredded chicken, sour cream, cream cheese, and chopped onions. Stir until everything is well mixed at a smooth, creamy consistency.

Grease a 9x13-inch casserole dish with cooking spray or butter. Spread the prepared chicken mixture into the bottom of the dish.

In a small bowl, crush the Ritz crackers into fine crumbs. Melt some butter and mix it into the crushed crackers until thoroughly combined. Sprinkle generously over the chicken mixture.

Preheat your oven to 350°F (175°C). Bake the casserole for approximately 30 minutes, or until the top is golden brown and the edges are bubbling.

Once baked, remove the casserole from the oven and let it cool for about 5-10 minutes before serving.