Hashbrown Breakfast Casserole

Name: Hashbrown Breakfast Casserole
Ingredients: 1 20-oz. package frozen hash brown patties; 2 Tbsp. extra-virgin olive oil; 1½ lb. bulk breakfast sausage; 1 large onion, finely chopped; ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt, plus more; 4 garlic cloves, coarsely chopped; 1 10-oz. package frozen chopped spinach, thawed in bag; 18 large eggs; 1 cup heavy cream; Freshly ground black pepper; 4 oz. extra-sharp cheddar cheese, coarsely grated