1. Pierce the meat all over with a fork and place in a large resealable plastic bag. Add 2 tablespoons of the olive oil, the Worcestershire, vinegar, garlic and whole rosemary. Seal the bag, pushing out all the air. Shake the bag a few times and refrigerate 8 hours or overnight.
2. Preheat the broiler and line a rimmed baking sheet with aluminum foil. Remove the meat from the marinade and pat dry. Sprinkle both sides with 1 1/2 teaspoons salt and 1/2 teaspoon pepper. Broil, flipping once, until the internal temperature reaches 125 degrees F, about 13 minutes. Transfer to a cutting board, tent with foil and let rest at least 15 minutes.
3. Meanwhile, heat the remaining 2 tablespoons oil in a large skillet over medium-high heat. Cook the bacon, stirring, until crispy, about 5 minutes. Add the mushrooms and cook until golden brown and tender, about 10 minutes. Add the breadcrumbs, butter, chopped rosemary and 1/4 teaspoon each salt and pepper. Cook, stirring, until the breadcrumbs are golden brown, another 3 minutes. Set aside.
4. Stir together the sour cream and horseradish and season with salt and pepper. Slice the steak against the grain, top with the mushrooms and garnish with the chopped parsley. Serve with the horseradish sauce on the side.
London broil can refer to several different cuts of beef from the top round or part of the flank steak. The overnight marinating should tenderize any of those cuts, but we found a London broil from the top round to be an especially tender and tasty cut for this dish.
Recipe courtesy Food Network Kitchens