01 - 
                Preheat oven to 175°C. Grease a 23 x 33 cm baking dish with nonstick spray.
              
              
              
                02 - 
                In a large bowl, mix thawed potatoes with melted butter, shredded cheddar cheese, and sour cream until thoroughly combined. Set aside.
              
              
              
                03 - 
                Melt butter in a non-stick skillet over medium heat. Add diced onion and cook until softened, about 3–5 minutes. Stir in minced garlic and cook for 1 minute longer.
              
              
              
                04 - 
                Stir in flour, kosher salt, black pepper, cayenne pepper, and dried mustard. Cook for another minute, then gradually add whole milk and chicken broth, whisking until smooth. Continue to cook, stirring, until the sauce thickens.
              
              
              
                05 - 
                Pour thickened sauce over the potato mixture. Stir well to combine and spread the mixture evenly in the prepared baking dish.
              
              
              
                06 - 
                Bake in the preheated oven for 45–50 minutes, until the casserole is heated through and bubbling.
              
              
              
                07 - 
                While casserole bakes, line a large baking sheet with two layers of paper towels. In a large saucepan, heat vegetable oil over high heat to 175°C.
              
              
              
                08 - 
                Working in batches, fry about 1 cup of onion rings at a time in the hot oil, stirring occasionally, until pale golden brown, about 2–5 minutes per batch. Remove with a slotted spoon and drain on prepared paper towels. Ensure oil returns to 175°C before each new batch.
              
              
              
                09 - 
                Sprinkle fried onions with kosher salt, tossing to coat evenly.
              
              
              
                10 - 
                After the casserole has baked, remove from the oven and top evenly with crispy fried onions. Serve hot.