To make the blue cheese dip, blend all the ingredients together except the parsley in a bowl with a hand blender, then stir in the parsley and season to taste. Cover with cling film, then place in fridge. This can be made up to 2 days in advance.
For the burger gently combine all ingredients in a large bowl, be careful not to over mix. Cover the bowl with cling film, place in fridge for 20 to 30 minutes. This can be made the night before.
Heat your grill (or grill pan) to medium-high heat.
Remove the bowl from the refrigerator, divide the meat into thirds, and form burgers. Season with sea salt on both sides.
Grill the burgers for 10-12 minutes per side (depending on the burger thickness)
Serve the burgers on the brioche buns, with some dressed mixed salad and blue cheese sauce.