Preheat your oven to 350 degrees Fahrenheit.
Warm up your leftover pot roast in its sauce that it cooked it. I did it on the stove top but you can easily transfer the meat and some of the juices to a microwave proof container.
To caramelize the onions, melt the butter in a pan. Add the sliced onions and cook on low-medium heat until the onions are cooked though and caramelized. Still the onions occasionally. You will be cooking the onions for about 12 minutes. Set the onions aside.
Place some nonstick foil on a baking sheet. This part will save you a lot of time cleaning.
Split the Kaiser rolls or French Rolls into two slices.
Add a teaspoon of the cream style horseradish to what will be the lower layer.
Top the lower layer of bread with a good helping of the caramelized onions.
Next, top that with a helping of the pot roast.
Arrange the cheese on top of the meat.
Top it all off with the top slice of bread.
Bake your sandwiches until the cheese just begins to melt and the bread gets slightly toasty.