Butter one side of each slice of bread and place butter side down on waxed paper.
Heat oil in skillet over medium heat. Warm beef in oil, gathered together as if on a sandwich. Top with two slices mozzarella cheese. Cover with lid until cheese is melted. Remove to a plate and keep warm.
Place one slice of bread, butter side down in skillet over medium heat. Using a spatula, move the meat and cheese onto the slice of bread in the skillet.
Top with pickles and dollop or drizzle horseradish mayo over the pickles.
Add second slice of bread, butter side up, onto the sandwich in the skillet.
Grill until golden brown, then flip the sandwich to grill the other side.
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Notes
We used horseradish mayo for a little kick. If you want it hotter/spicier, try horseradish sauce. For a milder approach, use plain mayo or mustard. Pickles add a nice crunch to the sandwich. You can substitute the pickles with coleslaw or pickle relish if preferred.
For the cheese, we use mozzarella but you can certainly swap it with Swiss cheese.
This sandwich is perfect for using up leftover corned beef. You can normally find pre-cooked corned beef in the deli section of your grocery store.
Sourdough bread goes very well with corned beef, but you can also use a marbled rye, light rye, or dark rye bread instead.