In a large mixing bowl, combine the beef mince, finely diced red onion, crumbled blue cheese, and fresh breadcrumbs. Add in the lightly beaten egg, parsley, Worcestershire sauce, Dijon mustard, salt, pepper, and a splash of Tabasco for heat. Mix gently but thoroughly until all ingredients are well incorporated.

Divide the mixture into six equal portions and shape each into a thick patty, about ¾ inch thick. Refrigerate for 30 minutes.

Heat a skillet over medium heat. Lay the slices of pancetta in the skillet and cook for about 2-3 minutes on each side until crisp. Transfer to a paper towel-lined plate.

In the same skillet, add oil if needed, and place the chilled burger patties. Cook over medium heat for 3-4 minutes on each side.

In a small bowl, combine crumbled blue cheese, sour cream, chives, olive oil, and Worcestershire sauce until smooth and creamy.

Toast the brioche buns lightly. Spread caramelized onion chutney on the bottom half of each bun. Assemble the burger with the patty, pancetta, and blue cheese sauce, then cover with the top half of the bun.