Roast Beef Sandwich with Mashed Potatoes

Instructions

1. 1

   Prepare the Horseradish Sauce

   • In a small bowl, whisk together the mayonnaise, prepared horseradish, Dijon mustard, and Worcestershire sauce.
   • Season with 1/4 teaspoon of salt and the black pepper. Cover and refrigerate while you prepare the other ingredients.
2. 2

   Caramelize the Onions

   • Melt the butter in a large skillet over medium-low heat.
   • Add the sliced onions and the remaining 1/4 teaspoon of salt. Stir to coat.
   • Cook slowly for 20-25 minutes, stirring every few minutes, until the onions are soft, sweet, and a deep golden-brown color. If they start to stick, add a tablespoon of water to deglaze the pan.
   • Once caramelized, remove the onions from the skillet and set aside.
3. 3

   Prepare the Sandwiches for Broiling

   • Adjust your oven rack so it is about 6 inches from the broiler element and preheat the broiler on high.
   • Slice the ciabatta rolls in half horizontally and place them cut-side up on a large baking sheet.
   • Generously spread the prepared horseradish sauce on the inside of all roll halves.
4. 4

   Layer the Ingredients

   • On the bottom half of each roll, pile approximately 6 oz of the thinly sliced roast beef.
   • Top the beef with a quarter of the caramelized onions.
   • Place two slices of provolone cheese over the onions, ensuring they cover the onions and beef.
5. 5

   Broil to Perfection

   • Place the baking sheet with the open-faced sandwiches under the preheated broiler.
   • Broil for 1-3 minutes, watching constantly, until the cheese is completely melted, bubbly, and just beginning to brown in spots. The top buns should be lightly toasted.
   • Do not walk away, as the broiler can burn the sandwiches in seconds.
6. 6

   Final Assembly and Serving

   • Carefully remove the baking sheet from the oven.
   • If using, place a small handful of fresh arugula on top of the melted cheese.
   • Place the top half of the roll onto the sandwich. For easier eating, slice in half diagonally with a serrated knife. Serve immediately.