In a skillet over medium heat, cook the bacon slices until crispy, about 6-8 minutes. Transfer to a plate lined with paper towels.

In the same skillet, remove excess grease and add sliced onions, sautéing for 10-15 minutes until they turn golden brown. Stir in Irish stout and season with salt and pepper.

Combine ground beef, Worcestershire sauce, salt, and pepper in a large bowl. Mix gently and divide into four equal portions, shaping into patties slightly larger than the buns.

Preheat grill or skillet over medium-high heat. Cook patties for 3-4 minutes on each side for medium-rare. In the last minute, add grated cheddar to melt.

Combine sour cream, Dijon mustard, and a pinch of salt in a small bowl until smooth to create the sauce.

Butter the cut sides of the brioche buns and toast face down on the skillet or grill for about 1-2 minutes until golden brown.

Assemble by placing a patty on the bottom bun, topping with cheddar, caramelized onions, bacon, and creamy sauce before capping with the top bun.

Serve hot, ideally alongside crispy fries or chips.