Funeral Potatoes

Name: Funeral Potatoes
Ingredients: 10 Tbsp. unsalted butter, melted, slightly cooled, divided, plus more for pan; 1 (30- or 32-oz. bag) frozen shredded hash browns, thawed; 1 small onion, finely chopped; 2 (10.5-oz.) cans cream of chicken soup; 1 oz. Parmesan, finely grated (about ½ cup); 1½ cups shredded Mexican cheese blend (about 6 oz.); 1 cup sour cream; 1½ tsp. Diamond Crystal salt or ¾ tsp. Morton kosher salt, divided; ½ tsp. freshly ground pepper; ½ tsp. ground dried sage; ¼ tsp. crushed red pepper flakes; 4½ cups cornflakes