Melt butter & oil in a large cast iron skillet over medium heat.

Add the onions. Stir every 5 minutes or so, making sure to scrape up all the good stuff on the bottom of the pan, until onions are completely brown. Reduce the heat if ends begin to blacken.

When the onions are a dark, rich brown, deglaze the skillet with whiskey and scrape the bottom until liquid has evaporated. Season with salt & pepper to taste and set aside.

Spread mayo on the outside of one slice. Top with a quarter of the cheese, half the onions, another quarter of the cheese, and another slice of bread with mayo on the top. Repeat for the second sandwich.

Grill sandwiches over medium heat in a large pan until bottoms are golden brown. Flip and grill until other side is golden and cheese is melted.