Begin by loosely forming ⅓ lb squares of ground beef, set onto a tray and place in the fridge to firm for 30 minutes. In another bowl, mix together all the ingredients for your sauce and set that in the fridge until you are ready.
Preheat your grill to medium high heat (around 350F) adding a cast iron skillet to the grill 2 minutes before cooking.
Melt some butter in the skillet, then add the thinly sliced white onions. Saute the onions for 10 minutes until they are beginning to brown, then add the brown sugar and bourbon (carefully). Let the onions simmer for another 4-5 minutes until really well softened, and pull off to cool.
Next, add the same cast iron skillet or a new one to the grill for the burgers. Pull your burger patties out of the fridge and add to the skillet. Cook for 2 minutes on the first side or until they begin to caramelize. Flip the patties over and gently “chop” the meat so that the ground beef patties loosen up and get a larger surface area. Cook for 2 minutes, then flip the patties back over, top each patty with some onions, cover with a slice of cheese and let melt. Pull the patties off and let cook for 2 minutes. While the patties cool, toast your Texas toast over the flames to a nice golden brown.
Start assembling your patty melt with a spoonful of the sauce on the bottom piece of, 1-2 patties and more sauce for the top piece of toast. Serve and enjoy!