Croissant Steak Sandwiches with Caramelized Onions and Horseradish Mayonnaise

Ingredients

• 2 cups arugula 
• 1 cup beef broth 
• 3  beef tenderloin steaks 1-inch-thick
• 4 tablespoons butter divided ()
• 4 large croissants lightly toasted halved
• 0.3 cup horseradish white prepared
• 0.5 cup mayonnaise 
• 3 medium onions thinly sliced
• 5 cups mushroom caps stemmed thinly sliced

Equipment

• bowl
• frying pan
• tongs

Directions

1. Mix mayonnaise and horseradish in small bowl to blend.
2. Melt 1 tablespoon butter in heavy large skillet over medium-high heat.
3. Sprinkle tenderloin steaks with salt and pepper.
4. Add steaks to skillet and cook to desired doneness, about 5 minutes per side for medium-rare. Using tongs, transfer steaks to work surface.
5. Melt 3 tablespoons butter in same skillet over medium-high heat.
6. Add onions; sauté until dark brown, about 25 minutes.
7. Add mushrooms; sauté until tender, about 5 minutes.
8. Add broth; boil until juices are reduced to glaze, stirring occasionally, about 1 minute. Season onion mixture to taste with salt and pepper.
9. Place 1 croissant bottom, cut side up, on each of 4 plates.
10. Spread each with horseradish mayonnaise and top with 1/2 cup arugula. Thinly slice steaks and divide among croissants. Top with onion mixture.
11. Spread remaining mayonnaise on cut side of croissant tops.
12. Place tops on sandwiches.