Ingredients

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

Directions

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 to work surface.

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.

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.