Preheat your oven to 350°F (175°C).
Steam the broccoli florets until just tender, about 4-5 minutes, then drain well.
In a medium bowl, whisk together the Knorr Creamy Chicken Soup Mix, water, and milk until smooth.
In a large mixing bowl, combine the cooked chicken, steamed broccoli, cooked rice, chopped onion, sour cream, mayonnaise, shredded cheddar cheese, and the prepared Knorr soup mixture. Stir well to combine.
Transfer the mixture into a greased 9×13-inch casserole dish and spread evenly.
In a small bowl, mix the crushed crackers with melted butter, then sprinkle evenly over the casserole.
Top the casserole with shredded mozzarella cheese.
Bake in the preheated oven for 35-40 minutes, or until the casserole is bubbly and the topping is golden brown.
Remove from oven and let it rest for 5 minutes before serving.