Preheat oven to 350° Fahrenheit. Spray baking dish with neutral cooking spray, or grease baking dish with refined coconut oil. Set baking dish aside.
Place 2 to 3 layers of paper towels on counter. Drain all liquid from 1 16-ounce bag shredded hashbrowns then transfer hashbrowns to paper towels. Cover hashbrowns with additional paper towels and squeeze as much liquid out of hashbrowns as possible. Repeat with fresh paper towels as needed until excess moisture has been removed. Set hashbrowns aside.
Add 1 pound Whole30-compliant ground breakfast sausage to large skillet and heat pan over medium-high heat. Cook sausage, stirring frequently to break into small pieces, until sausage is browned and crumbled, approximately 5 minutes.
If soaked cashews have not yet been blended into cashew cream, blend cashews now, then set aside.
Sprinkle 2 tablespoons arrowroot powder evenly over browned sausage. Stir well until sausage crumbles are fully coated in arrowroot powder.
Pour 1 ½ cups of prepared cashew cream into skillet and stir to incorporate. Reserve remaining cashew cream for later.
Cook sausage and cashew cream, stirring frequently, until mixture thickens and begins to boil, approximately 3 to 5 minutes. Sprinkle plenty of freshly cracked black pepper over mixture and stir to incorporate, then reduce heat under skillet to low and let mixture simmer.
While mixture simmers, add 6 large eggs, remaining cashew cream, ½ teaspoon garlic powder, ½ teaspoon onion powder, ½ teaspoon salt, and one third of chopped green onions to medium mixing bowl. Whisk well until ingredients are fully incorporate, then set bowl aside.
Spread hashbrowns evenly across bottom of baking dish, covering bottom completely. Sprinkle salt over hashbrowns to taste, then evenly sprinkle one third of chopped green onions over top of hashbrown layer.
Spread entire sausage mixture evenly over top of hashbrowns and green onions, making sure to cover hashbrowns completely.
Top sausage mixture with entire egg mixture, making sure to distribute egg mixture evenly across casserole dish.
Place baking dish in preheated oven. Bake casserole 40 to 50 minutes or until surface of casserole is firm and eggs are cooked.
When casserole is ready, carefully remove baking dish from oven and sprinkle remaining third of chopped green onions evenly over top of casserole. Set baking dish aside and let casserole rest 3 to 5 minutes, then cut casserole into equally-sized squares and serve warm.