Preheat the oven to 375 degrees F. Generously coat a 9x13-inch baking dish with nonstick spray or butter it.
Heat a large, nonstick skillet over medium-high heat. Break the sausage up in the skillet into small pieces and brown it, until it is fully cooked through. With a slotted spoon, remove to a large mixing bowl. No need to wipe the skillet clean.
Drizzle in the olive oil. Add the bell pepper and three-fourths of the green onions. Stir and cook until the pepper is crisp-tender, about 8 minutes.
Add the spinach a few handfuls at a time, stirring until the spinach wilts. It will look like a lot of spinach at first, but it will cook down considerably. This will take about 2 minutes. Add the vegetables to the mixing bowl with the sausage.
To the mixing bowl, add the hash browns and ½ cup of the cheese. With a large spoon, gently stir to combine.
In a separate larger mixing bowl, whisk together the eggs, milk, hot sauce, salt, and pepper until blended.
Pour the egg mixture over the hash brown mixture and stir gently to combine. Carefully pour into the prepared baking dish. Use a spoon to prod the filling into an even layer. Top evenly with the remaining ½ cup cheese.
Cover the casserole with aluminum foil and bake for 30 minutes.
Remove the foil and continue baking until the eggs are set and the potatoes are tender (I use a fork to remove a potato and test it), about 15 to 20 additional minutes. Let rest a few minutes. Sprinkle with the reserved green onions. Slice and enjoy!
TO STORE: Refrigerate casserole in an airtight storage container for up to 3 days.
TO REHEAT: Gently rewarm leftovers in a baking dish in the oven at 350 degrees F.
TO FREEZE: Freeze casserole in an airtight freezer-safe storage container for up to 3 months. Let thaw overnight in the refrigerator before reheating.
TO MAKE AHEAD: Fully assemble the casserole up to 1 day in advance, and store it unbaked and covered in the refrigerator. When you're ready to bake, let the casserole come to room temperature while you preheat the oven, then bake it as directed.