Combine the horseradish and mustard in a small bowl, and then set aside.
Butter one side of each piece of bread.
Spread the horseradish mixture over the unbuttered side of the bread.
To build the sandwich, add the pastrami, cheese, dill pickle and then caramelized onions between the two slices of bread.
Place one side of the buttered sandwich on a cold skillet.
Turn on the heat to medium low and slowly cook the sandwich until the cheese softens and the bread is golden about 3-5 minutes.
Carefully flip the sandwich over to finish cooking.
Again, slowly cook the sandwich over medium low heat for another 3 minutes. Don’t try to speed up the process by cooking over a higher heat otherwise the bread will burn and the cheese won’t melt.
Once the sandwich has finished cooking, remove from the heat and serve immediately.