01 -
Preheat oven to 350°F. Spray a 9×13-inch baking dish with nonstick spray.
02 -
In a large bowl, combine the potatoes, butter, shredded cheese, and sour cream. Mix well and set aside.
03 -
In a medium, non-stick skillet over medium heat, melt butter. Add onions and cook until softened (3-5 minutes). Add the garlic and cook for one more minute.
04 -
Add flour, salt, pepper, cayenne pepper, and dry mustard. Cook another minute. Slowly add milk and chicken broth, whisking until well combined. Continue to cook, whisking until thickened.
05 -
Pour the sauce into the potato mixture. Mix well. Then, pour into the prepared baking dish. Bake 45-50 minutes, until the casserole is bubbly and heated through.
06 -
While the casserole is baking, line a large baking sheet with two layers of paper towels. To a large saucepan over high heat, add oil and heat to 350°F. Working in batches, add about 1 cup of onions to the hot oil. Fry, stirring occasionally, until onions turn pale golden brown, about 2-5 minutes. Carefully remove the fried onions and place on the lined baking sheet. Repeat with remaining onions, ensuring the oil is at 350°F before frying each batch.
07 -
Sprinkle fried onions with salt, tossing to coat.
08 -
After the casserole has baked for 45-50 minutes, remove from the oven. Sprinkle the fried onions evenly over the top of the hot casserole. Serve immediately.