Spread mayonnaise on one side of each slice of bread.
Place one of the slices, mayo side down, on a clean plate or work surface. Spread mustard onto slice and top with one slice of Havarti and one slice of Emmental.
Top with caramelized onions evenly spread over cheese, then bacon, then one more slice of Havarti.
Place second slice of bread on top with mayo facing up.
Heat a skillet over medium-high heat.
Place sandwich in skillet to cook for 3-4 minutes each side, or until deep golden brown and cheese is melted.
Caramelized onions: I like this method for making caramelized onions by The Kitchn, and they can be used as toppings and ingredients for so many dishes! You can also buy prepackaged caramelized onions.Cheese Alternatives: If you can't find Havarti, some alternatives are Gouda, or Monterey Jack. If you can't find Emmental, try Jarlsberg, Gruyère, or Swiss.