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Melt the butter in a frying pan over a medium heat until foaming, then fry the shallots with a pinch of salt for 10 mins until softened and translucent. Set aside to cool.
Tip the beef mince into a large bowl, then add the cooled shallots, the breadcrumbs, egg yolk, mustard, pepper and 1 tsp salt. Squish the mixture together using your hands until well-combined. Divide into four equal portions, weighing for accuracy, if you like, then form these into four burger patties. Keep covered in the fridge. Will keep chilled for up to 48 hrs.
Heat the grill to medium. Arrange the pancetta on a baking tray and grill for 3-4 mins until crisp.
Meanwhile, for the blue cheese sauce, combine the cheese, mayo and soured cream together in a bowl. Season to taste and set aside.
Remove the pancetta from the grill and heat the oven to 220C/200C fan/gas 7. Heat the oil in a large frying pan over a medium heat and cook the burgers for 3 mins on each side until browned. Transfer to a baking tray and finish cooking in the oven for 10-12 mins until cooked through. Cover and leave the burgers to rest for 5 mins.
Toast the cut sides of the buns in a dry frying pan. Spread the bases with the cranberry sauce, lay the patties on top, add the blue cheese sauce, salad leaves and pancetta, then finish with the bun tops.