Preheat oven to 350 degrees F (175 degrees C). Grease a 2-quart casserole dish.
Heat vegetable oil in a skillet over medium heat. Sprinkle the chicken breasts with salt, black pepper, and 1 teaspoon of salt-free seasoning, and pan-fry in the hot oil until lightly browned on both sides, 6-8 minutes per side. Cut the chicken breasts into bite-size pieces, and set aside.
Chop broccoli and onion and steam until just tender.
Make rice side dish.
Transfer cooked rice into a large mixing bowl, and stir in the chicken, broccoli, and onion.
Mix in the cream of chicken soup, mayonnaise, 2 cups of Cheddar cheddar, 2 teaspoons of salt-free seasoning, garlic powder, and Dijon mustard until thoroughly combined.
Adjust salt and black pepper.
Spoon the mixture into the prepared casserole dish, and sprinkle 1/2 cup of Cheddar cheese over the top.
Bake in preheated oven until the casserole is bubbling and lightly browned, about 30 minutes.