Preheat the oven to 450 degrees . Tear the mushrooms into long thin strips. On a large rimmed baking sheet, toss the mushrooms and shallots with the sherry and EVOO. Season with salt and pepper and roast until tender, 15 minutes. Toss and continue roasting until browned and crispy, 3 to 5 minutes more.
Meanwhile, heat a cast-iron skillet over medium-high. Add the butter; when it stops foaming, season the beef and add to the skillet. Cook until browned and cooked to desired doneness, about 3 to 4 minutes per side for medium-rare. Transfer to a plate, tent with foil and let rest 10 minutes.
In a small bowl, stir the creme fraiche, chives, horseradish, lemon juice and Worcestershire; season.
Slice the beef and divide among plates. Dollop creme fraiche on top; garnish with chives and pepper. Serve with a pile of the shoestring mushrooms.