In a large, deep skillet, heat oil and add onions. Cook on medium heat, stirring frequently for 15 minutes or until the onions are a light golden brown. Season the chicken cubes with salt and pepper, then add the chicken to the onions and cook until the chicken is browned. (The onions will continue to deepen in color finishing the caramelization process while the chicken cooks.)
Add the sour cream and cream of chicken soup to the chicken and onions. Stir until heated through.
Melt butter in a separate bowl, and add the Ritz cracker pieces and poppyseeds. Stir well.
Pour chicken mixture into a 13x9-inch baking dish and sprinkle the cracker mixture over the top.
Bake at 350 degrees Fahrenheit for 20 to 30 minutes or until bubbly and heated through. Serve over rice.