Croissant Steak Sandwiches with Caramelized Onions and Horseradish Mayonnaise

Ingredients

• 8 ounces large shiitake mushrooms,
• ¼ cup prepared white horseradish
• ½ cup mayonnaise
• 4 tablespoons butter
• 3 1-inch-thick beef tenderloin steaks
• 3 medium onions
• 1 cup beef broth
• + 2 more ingredients
• • 4 large croissants
  • 2 cups arugula

Mix mayonnaise and horseradish in small bowl to blend. Melt 1 tablespoon butter in heavy large skillet over medium-high heat. Sprinkle tenderloin steaks with salt and pepper. Add steaks to skillet and cook to desired doneness, about 5 minutes per side for medium-rare. Using tongs, transfer steaks...

View full recipe at Epicurious