Preheat your oven to 350°F. Spray a 9x13 baking dish with nonstick cooking spray.
Add the butter to a deep skillet or pot and melt it over medium heat. Once the butter begins to melt, add the diced onions and saute them until they are softened (about 5 minutes).
Gently sprinkle in the flour, and stir it around until all of the butter is absorbed. Continue simmering the butter and flour mixture for about 3-5 minutes, to get the flour taste out.
Now pour in your heavy cream and milk and whisk it around until the flour is smooth and combined into the sauce.
Sprinkle in all of your seasonings including the seasoned salt, black pepper, garlic powder, and onion powder. Stir the sauce up well so that the seasonings are fully incorporated.
Reduce the heat to medium-low. Now add in the American cheese, and cheddar cheese. Mix the cheese in well and continue stirring until the cheese is fully melted. Remove from the heat and set it aside.
Bring a pot of water to a boil. Once it is boiling, add the broccoli pieces into the water and cook for two minutes. Remove the broccoli from the boiling water immediately, as it will continue cooking in the oven.
Add rice to a large bowl. Pour the cheese sauce into the bowl and stir it up until everything is fully incorporated. Next, gently fold the broccoli florets into the cheesy rice mixture. Careful not to break the broccoli pieces up too much.
Pour the cheddar broccoli rice mixture into your pre-greased baked dish. Spread it out evenly. Now sprinkle the remaining cheddar cheese over the top of the dish. Sprinkle with the paprika for additional flavor and color.
Bake in the preheated oven for 20 minutes, until the cheese is melted and bubbly. To brown the cheese even more, set the oven to broil for 3 minutes. (Keep a close eye so that the cheese does not burn.)
Remove from the oven.
Let the broccoli rice casserole sit for at least 5 minutes before serving. Enjoy!