Preheat oven to 350 degrees F (175 degrees C). Spray a 9×12-inch baking dish with cooking spray.
Place the frozen hash brown potatoes, half-and-half cream, butter, onion, salt, black pepper, and onion salt into a saucepan over medium heat. Cook the mixture, stirring occasionally, until the frozen potatoes are thawed and the ingredients are well mixed. Pour the potato mixture into the prepared baking dish.
Bake in the preheated oven until the casserole is golden brown on top, about 1 hour.
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