Heat a large skillet over medium heat. Add the bacon strips and cook for about 5–7 minutes, flipping once, until crispy and golden. Remove and drain on paper towels.

In the same skillet with the reserved bacon drippings, add thinly sliced onions and cook over medium heat for about 10–12 minutes until caramelized. Pour in the Irish stout and simmer for another 5 minutes.

In a mixing bowl, combine ground beef, Worcestershire sauce, salt, and pepper. Gently mix and form into patties slightly larger than the buns.

Preheat a grill or skillet over medium-high heat. Cook patties for 3–4 minutes on each side until medium doneness. Lay a slice of cheese on each patty in the last minute of cooking.

In a small bowl, mix sour cream, Dijon mustard, and a pinch of salt until smooth. Set aside.

Butter the inside of each brioche bun half and toast in the skillet for 2–3 minutes until golden.

Assemble burgers with a beef patty on each bun bottom, followed by Guinness-caramelized onions, bacon, and sauce before topping with the bun.