Balsamic Marinated Steak and Unstuffed Mushrooms

Name: Balsamic-Marinated Steak and Unstuffed Mushrooms
Ingredients: One 2 to 2 1/2-pound 1 1/2-inch-thick London broil; 1/4 cup olive oil; 1/4 cup Worcestershire sauce; 2 tablespoons balsamic vinegar; 4 cloves garlic, smashed; 2 whole sprigs rosemary plus 1 teaspoon chopped leaves; Kosher salt and freshly ground black pepper; 1 strip bacon, chopped (about 1/2 ounce); 1 pound cremini mushrooms, halved; 1/2 cup panko breadcrumbs; 2 tablespoons unsalted butter; 1/2 cup sour cream; 2 tablespoons prepared drained horseradish; 1 tablespoon chopped fresh parsley, for garnish