Cook the bacon in a skillet over medium heat for 6-8 minutes until crispy. Transfer to paper towels to drain excess grease.

Remove excess grease from the skillet, add sliced onion, and cook over medium heat for 10-15 minutes until caramelized. Pour in Irish stout and simmer for 5 minutes until reduced by half. Season with salt and pepper.

In a mixing bowl, combine ground beef with Worcestershire sauce, salt, and pepper. Form into 4 equal patties.

Heat the grill or skillet over medium-high heat. Cook patties for 3-4 minutes on one side. Flip and add cheddar cheese on top, cover to melt for 1 minute.

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

Melt butter in a skillet over medium heat. Toast brioche buns cut side down for 2-3 minutes until golden.

Assemble the burgers with the bottom bun, patties, caramelized onions, bacon, and creamy mustard sauce. Top with the other half of the bun.

Serve hot with sweet potato fries or chips.