Au Gratin Hash Brown Casserole

Ingredients

• 1 package frozen shredded hash brown potatoes
• 1 cup sour cream
• 1 1/2 Tbsp chopped fresh parsley
• 1/2 cup finely chopped yellow onion
• Salt and freshly ground pepper
• 1 1/2 cups shredded sharp cheddar cheese
• 3 Tbsp butter
• 1/4 cup all-purpose flour
• 1/2 cup finely shredded parmesan cheese
• 4 Tbsp butter
• 4 cups cornflakes
• 2 3/4 cups milk