Sausage Hash Brown Breakfast Casserole

Name: Sausage Hash Brown Breakfast Casserole
Ingredients: 1 1/2 tablespoons olive oil (divided); 450 g sausages de-cased, choose any of Italian, turkey chicken, breakfast or plant based; 1 teaspoon garlic powder; 1 teaspoon oregano; 1 teaspoon paprika; 1 medium onion (diced, about 1 ½ cups); 2 garlic cloves (minced); 2 medium red bell peppers (diced); 800 g bag frozen diced hash brown potatoes (do not thaw); 2 teaspoons kosher salt (divided 1 teaspoon each); 1 teaspoon freshly ground black pepper (divided ½ teaspoon each); 3 cups shredded cheddar cheese (divided 2 cups and 1 cup); 1/2 cup sliced green onions (plus more for garnish); 10 eggs; 3/4 cup milk; 2 teaspoons crushed red pepper flakes (as desired)