Preheat your griddle or your cast-iron skillet over medium heat. In my case, I am using the Cuisinart 3-in-1 Pizza Oven Plus.
Thinly slice the onion, and set aside while you add the olive oil onto the griddle surface.
Cook the onions, stirring occasionally, for about 20 minutes or until they are a deep brown color.
Near the end of cooking, add one tbsp of apple cider vinegar and stir well to combine. Remove, reserve and set the heat of your griddle to medium-high heat.
Add the sriracha and maple syrup to the ground beef and shape into two thin patties that are roughly the shape of your bread slices. Generously season the patties with salt and pepper.
Cook the patties for about 5 minutes per side or until they are fully cooked.
Brush one side of each bread with butter and lay two slices on the griddle, add a slice of cheese, the burger patty, half of the onion, another slice of Swiss cheese and finish with the last slice of bread.
Press the sandwich, cook for one minute on each side then remove.
Whisk together eggs, milk, garlic powder, cinnamon powder, sriracha, vanilla and dip both sides of the sandwich in the mixture.
Add some oil to the griddle and fry the sandwich gently on both sides until the cheese has melted.
Serve hot and enjoy with a fork.