In a medium skillet, melt 1 tbsp unsalted butter over medium heat. Add the thinly sliced yellow onion and cook, stirring occasionally, until softened and translucent, about 8-10 minutes.

Pour in 1/2 cup Guinness stout, 1 tbsp light brown sugar, 1 tbsp balsamic vinegar, and 1/4 tsp salt. Bring to a simmer, then reduce heat to low and cook, stirring occasionally, until the liquid has mostly evaporated and the onions are deeply caramelized and tender, about 10-15 minutes. Set aside.

In a small saucepan, melt 2 tbsp unsalted butter over medium-low heat. Add 2 tbsp all-purpose flour and whisk constantly for 1 minute to create a roux.

Gradually whisk in 1.5 cups warm whole milk until smooth and no lumps remain. Continue to whisk as the sauce thickens, about 3-5 minutes.

Remove from heat and stir in 1.5 cups grated sharp cheddar cheese, 2 tbsp Irish whiskey, 1 tsp Dijon mustard, 1/4 tsp salt, and 1/8 tsp black pepper until the cheese is completely melted and the sauce is smooth and creamy. Keep warm over very low heat or gently reheat just before serving.

In a large mixing bowl, gently combine 1.5 lbs ground chuck with 1 tsp Worcestershire sauce, 1/2 tsp garlic powder, 1/2 tsp onion powder, 1 tsp kosher salt, and 1/2 tsp black pepper. Do not overmix, as this can make the burgers tough.

Divide the mixture into 4 equal portions and form them into patties, about 3/4-inch thick and slightly wider than your buns. Make a slight indentation in the center of each patty to prevent bulging during cooking.

Heat a large skillet or cast-iron grill pan over medium-high heat. Once hot, add the burger patties. Cook for 4-5 minutes per side for medium-rare, 5-6 minutes per side for medium, or longer for desired doneness.

While the burgers are cooking, in a separate skillet (or after removing burgers), cook the 8 slices of Irish back bacon (or thick-cut bacon) over medium heat until crispy, about 3-5 minutes per side. Drain on a paper towel-lined plate.

Lightly butter the cut sides of the 4 brioche burger buns and toast them in the burger skillet for 1-2 minutes until golden brown.

To assemble, spread a generous amount of Whiskey-Cheddar Sauce on the bottom bun. Layer with green leaf lettuce, a tomato slice, a cooked burger patty, a spoonful of Guinness Stout Onions, 2 slices of crispy bacon, and then the top bun. Serve immediately.