Croissant Steak Sandwiches With Caramelized Onions and Horseradi

• cup mayonnaise↗1⁄2
• cup white horseradish, prepared↗1⁄4
• tablespoons butter (1/2 stick, divided)4
• beef tenderloin steaks, thick↗3
• medium onions, thinly sliced↗3
• ounces shiitake mushrooms, stemmed, caps thinly sliced (about 5 cups)↗8
• cup beef broth↗1
• large croissants, halved horizontally, lightly toasted↗4
• cups arugula↗2

Step-by-step instructions

1. 1

   Mix mayonnaise and horseradish in small bowl to blend.

2. 2

   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 to work surface.

3. 3

   Melt 3 tablespoons butter in same skillet over medium-high heat. Add onions; sauté until dark brown, about 25 minutes. Add mushrooms; sauté until tender, about 5 minutes. Add broth; boil until juices are reduced to glaze, stirring occasionally, about 1 minute. Season onion mixture to taste with salt and pepper.

4. 4

   Place 1 croissant bottom, cut side up, on each of 4 plates. Spread each with horseradish mayonnaise and top with 1/2 cup arugula. Thinly slice steaks and divide among croissants. Top with onion mixture. Spread remaining mayonnaise on cut side of croissant tops. Place tops on sandwiches.