Preheat oven to 450°. Line the bottom of a 13x9" baking dish with one 20-oz. package frozen hash brown patties, trimming to fit. Bake until golden brown and crisp, 15–20 minutes. Remove from oven; reduce oven temperature to 350°.

Meanwhile, heat 2 Tbsp. extra-virgin olive oil in a large skillet over medium-high. Cook 1½ lb. bulk breakfast sausage, stirring and breaking up with a wooden spoon into marble-size pieces, until browned and just cooked through, 6–8 minutes. Using a slotted spoon, transfer to a small bowl.

Add 1 large onion, finely chopped, to same pan and season with kosher salt. Cook, stirring occasionally, until slightly softened and starting to brown, about 3 minutes. Add 4 garlic cloves, coarsely chopped; cook, stirring often, until fragrant, about 1 minute. Add one 10-oz. package frozen chopped spinach, thawed in bag, and any water in bag (this will help deglaze the pan); cook, stirring occasionally and scraping up any browned bits, until dried out, about 2 minutes. Remove from heat.

Whisk 18 large eggs, 1 cup heavy cream, and ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt in a large bowl; season generously with freshly ground black pepper. Stir in sausage, onion mixture, and 4 oz. extra-sharp cheddar cheese, coarsely grated; pour egg mixture over hash browns. Bake casserole until set and lightly browned on top, 40–50 minutes. Let cool slightly. Do Ahead: Hash brown breakfast casserole can be baked 1 month ahead. Cut into pieces; wrap individually in plastic wrap and freeze. Unwrap and reheat on a baking sheet in a 350° oven 15 minutes.