Preheat oven to 375˚F.
Spread hashbrowns on a baking sheet and roast them to "thaw" and crisp up while you prepare the sausage and veggies.
Place a skillet over medium heat. Add ½ tablespoon oil and brown sausage, breaking it up as it cooks. (If using cooked bacon, cook until crispy, then dice into bite-size pieces. Keep a tablespoon or so of bacon grease for flavor.)
Drain sausage and transfer to a large mixing bowl. Discard any excess fat and liquid from the pan.
Add 1 ½ tablespoons of oil to the skillet and sauté onions & bell pepper until vegetables just begin to soften. (About 5 minutes.) Season with just a pinch of salt, seasoning salt, and pepper, and stir it all together.
Transfer the vegetables to a bowl with the sausage or bacon. (See above in the post for alternate directions using bacon.)
Add the roasted hashbrowns, about ⅔ of the cheese, ⅔ of the green onions, and the remaining salt, pepper, and seasoned salt to the sausage and vegetable mixture and gently toss together until fully combined. (It's okay if the hashbrowns are a little warm - the cheese is going to melt anyway.)
Pour this back into the skillet or into a greased 13x9 pan.
In a mixing bowl, whisk together eggs and milk and pour this mixture over the sausage/vegetable mixture. Press the hashbrowns down into the milk/eggs to help it seep into all the nooks and crannies.
Sprinkle the top with the remaining cheese and a handful of green onions.
Cover casserole tightly with foil and bake for 35 minutes. Remove the foil and continue to bake for 10 more minutes or until the eggs set.
Serve immediately with salt and pepper to taste and some extra green onions.