For an extra layer of flavor, try using a mix of different cheeses, such as Gruyere or Monterey Jack. If you don't have buttermilk on hand, you can substitute with regular milk mixed with a tablespoon of lemon juice or vinegar. The caramelized onions are key to the depth of flavor in this casserole, so don't rush the process. Allow them to develop a rich, golden-brown color. Feel free to adjust the amount of sour cream and onion chips to your liking. Some people prefer a more subtle flavor, while others enjoy a bolder taste. This casserole can be assembled ahead of time and baked just before serving. Simply cover it tightly with plastic wrap and refrigerate for up to 24 hours.