6 medium blue potatoes, peeled and quartered
Salt and pepper
1/2 cup cream
1/4 cup chives, finely chopped
2 egg yolks
8 slices bacon, chopped
2 pounds ground sirloin
1/4 cup Worcestershire sauce
4 cloves garlic, minced or grated
1/4 cup grated onion, with juice
1/4 cup flat leaf parsley, finely chopped
Extra virgin olive oil (EVOO), for drizzling
2 tablespoons butter
1 onion, thinly sliced
3/4 pound crimini mushrooms, sliced
1/4 cup Stilton or other piquant blue cheese, crumbled
4 cups watercress leaves, loosely packed
4 rolls, split, lightly toasted and buttered
Balsamic drizzle, for garnish (optional)
Pre-heat the broiler.
Place the potatoes in a saucepot, cover with water and bring to boil. Add some salt and cook until tender. Drain the potatoes and return them to the hot pot. Add the cream, salt and pepper and chives and mash. Scoop the potatoes into a cake frosting piping bag or large zip-top sealable plastic bag with a star tip, if you have one (you can also cut off the corner of the plastic bag), and pipe them into little mounds on a baking sheet. Place under the broiler until golden brown on top, about 2 minutes.
Heat a cast-iron skillet over medium-high to high heat. Add the bacon and cook until crisp, about 3-4 minutes. Remove the bacon bits and reserve on a paper towel-lined plate. Remove all but a tablespoon of the bacon fat from the skillet.
In a large bowl, combine the 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 four portions, then form patties out of each portion, making them thinner in the middle. Drizzle the patties with EVOO, then add them to the hot skillet and cook for 10 minutes for rare to medium-rare and up to 16 minutes for medium-well burgers, turning once. Top the burgers in the last minute of cook time with the Stilton cheese and tent with foil to melt.
While the burgers cook, heat a small skillet over medium heat and melt the butter. Add the onion, sauté for 8-10 minutes, then add the mushrooms and sauté 8-10 minutes more. If the pan gets dry, add a drizzle of EVOO. Season the 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.