Preheat oven to 350 F degrees.
Thinly slice 2 scallions, both the whites and the greens.
In preheated medium sauce pot, add butter and stir to melt. Add Flour and whisk to combine with the butter. Cook stirring constantly for 2-3 minutes. Slowly whisk in the broth and then milk until the roux is completely and smoothly mixed into the liquids. Continue to stir and cook over medium heat until it comes to a light simmer and thickens. Turn off the heat.
To a large mixing bowl, add the frozen hash browns, the thickened broth mixture, sour cream, cheese, scallion, oregano, salt, and pepper. Mix to combine.
Add the hash brown mixture to a 9x13 casserole dish. Bake in the oven center rack for 45 minutes covered with aluminum foil. Remove the cover for the last 10 minutes and turn the broiler on to brown the top. Monitor the casserole to ensure that it does not burn under the broiler.
Serve and Enjoy!