Thinly slice your roast beef and toss in the thickened braising liquid in a Dutch oven over medium heat.
Line a baking sheet with aluminum foil and preheat your broiler.
Toast two pieces of bread and spread a thin layer of horseradish on one side. Place the toasted bread horseradish side up on your baking sheet.
Layer sliced roast beef onto horseradish to your desired height.
Place a slice of Swiss cheese on top of each open-faced sandwich and place them under the broiler until the cheese is melted.
While the sandwich is broiling, toast two more slices of bread and spread a thin layer of mustard on one side of each.
Remove sandwiches from the oven and top with the second slice of bread, cut in half, and serve immediately.
Notes
Flavor Tips - When choosing your horseradish know that prepared horseradish is fresh horseradish mixed with vinegar, while cream-style horseradish is mixed with sour cream or heavy cream.Storage - Roast beef melts are best when served right away. However, you can store leftover beef melts in an airtight container in the fridge for up to 4 days.Yield - 2 Sandwiches