Put all the ingredients in a large bowl and thoroughly mix together with your hands. Form 6 thick burgers, cover with cling film and chill for 30 minutes.
Heat a skillet or frying pan and cook the pancetta until crisp. Set aside.
Place the burgers on the pan and cook over medium heat, pressing down on the burgers with a spatula. Cook for 3-4 minutes on each side. Set aside while you prepare the blue cheese sauce.
Mix all the ingredients for the sauce together in a small bowl. Add more sour cream if the sauce is too thick.
Toast the burger buns and spread onion chutney on the bottom half. Add the burger and a pancetta slice. Spread blue cheese sauce on the top half of the bun and press down on the burger. Serve immediately.