Preheat broiler.
Place potatoes in a saucepot, cover with water and bring to boil. Add some salt and cook until tender. Drain potatoes and return them to the hot pot. Add cream, salt and pepper, chives and mash. Scoop potatoes into a piping bag or large zip-top baggie with a star tip if you have one, and pipe them into little mounds on a baking sheet. Place under broiler until golden brown on top, about 2 minutes.
Heat a cast-iron skillet over medium-high to high heat. Add bacon and cook until crisp, about 3-4 minutes. Remove bacon bits and reserve on a paper towel-lined plate. Remove all but a tablespoon of bacon fat from the skillet.
In a large bowl, combine meat with reserved bacon bits, Worcestershire, garlic, grated onion (grate it over the bowl so the juices fall into the meat), parsley, salt and lots of black pepper. Score the meat into 4 portions, then form patties out of each portion, making them thinner in the middle. Drizzle patties with EVOO, add them to the hot skillet and cook 10 minutes for rare to medium-rare and up to 16 minutes for medium-well burgers, turning once. Top burgers in the last minute of cook time with the stilton and tent with foil to melt.
While burgers cook, heat a small skillet over medium heat and melt butter. Add onions, saut 8-10 minutes then add mushrooms and saut 8-10 minutes more. If the pan gets dry, add a drizzle of EVOO. Season cooked onions and mushrooms with salt and pepper.
To serve, pile some watercress on the bun bottoms and top with a burger patty. Pile some of the onions and mushrooms on top and replace the bun caps. Serve the mashed blue potatoes alongside.