Roast Beef Sandwich

In a small mixing bowl, combine mayonnaise, prepared horseradish, Dijon mustard, Worcestershire sauce, garlic powder, and black pepper to create the horseradish sauce. Whisk until smooth and set aside. In a large skillet, heat olive oil over medium heat. Add the sliced red onion and cook for 3-4 minutes until softened and lightly caramelized. Pour in the beef broth and bring to a gentle simmer. Add the sliced roast beef to the warm broth and heat for 2-3 minutes, turning gently with tongs to warm through without overcooking. Season with salt and pepper to taste.

While the beef is warming, butter one side of each bread slice with the softened butter. Heat a large skillet or griddle over medium heat and toast the bread slices butter-side down for 2-3 minutes until golden brown and crispy. Flip and toast the other side for another 1-2 minutes. Remove from heat and immediately place provolone cheese slices on 4 of the toasted bread slices while they're still hot, allowing the cheese to begin melting. Spread the prepared horseradish sauce on the remaining 4 bread slices. Arrange the tomato slices and lettuce leaves on a clean cutting board for easy assembly.

Using tongs, remove the warmed roast beef from the skillet, allowing excess jus to drain back into the pan. Reserve the warm jus for serving. Layer the warm roast beef generously on the cheese-topped bread slices, followed by the caramelized onions, tomato slices, and lettuce leaves. Top with the horseradish sauce-spread bread slices to complete the sandwiches. Press gently and cut each sandwich diagonally with a sharp knife. Strain the reserved beef jus into small bowls for dipping. Serve immediately while the beef is warm and the cheese is melted, garnished with fresh chives. The combination of tender beef, tangy horseradish, and savory jus creates the perfect deli-style experience.