Place potatoes in a pot, cover with cold water and bring up to a boil. Season water with salt and boil potatoes until tender, 12 to15 minutes. Heads up: save a ladle of starchy cooking water just before draining.
Heat a skillet with high sides over medium high heat. Add extra-virgin olive oil, half a turn of the pan and the bacon. Crisp bacon and remove to paper towel lined plate. Add sirloin to the pan and caramelize the meat, 4 to 5 minutes. Add in the mushrooms and onions and cook until tender, 6 to 7 minutes more then season with salt and pepper.
While meat cooks heat a small sauce pot over medium heat and melt butter, whisk the flour into butter, cook 2 minutes then whisk beef stock into flour, add Worcestershire and season sauce with salt and pepper, to taste. Thicken 6 to 7 minutes.
Pour gravy over meat and turn on broiler.
Temper egg yolk by beating it with the starchy potato cooking water. Place drained potatoes back into the pot you cooked them in to dry them out a little. Mash potatoes with egg yolk and sour cream then fold in crumbled blue cheese and chives. Season the potatoes with salt and pepper and spread across the top of the meat in an even layer. Garnish the potatoes with paprika and place under broiler to crisp and brown the potatoes, 2 to 3 minutes. Crumble reserved bacon over top. Serve immediately right from the hot skillet.