Balsamic-Marinated Steak and Unstuffed Mushrooms

INGREDIENTS

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