Heat oven to 350 degrees F. Lightly coat a 13" x 9" baking pan or casserole dish with cooking spray.
In a large mixing bowl, toss together the thawed hash browns, scallions and cheese. Set aside.
In a large saucepan over medium-low heat, melt the butter. Stir in the sweet rice flour, salt, pepper and paprika. Slowly whisk in the milk, constantly stirring to prevent lumps. Continue to cook and stir the milk mixture until thickened and bubbly, about 5 to 10 minutes.
Pour the milk mixture into the hash brown mixture in the mixing bowl and stir well, until all of the hash browns are coated in the white sauce.
Transfer hash browns into the prepared baking pan and spread out evenly. Bake for 1 hour, or until casserole is bubbling and starting to brown on top and around the edges. Serve.