Add the mayonnaise and horseradish to a bowl, and mix well. Taste and adjust the amounts as necessary, then set aside.
1 tablespoon mayonnaise, ½ tablespoon horseradish
Add the oil to a skillet over medium heat. Once the oil is nice and hot, add sliced corned beef and heat through, about 3-5 minutes.
½ tablespoon olive oil, 5 ounces corned beef
Top each slice of corned beef with Swiss cheese, then cover the pan with a lid until the cheese is melted. Once melty, set the beef and cheese aside on a plate.
2 slices Swiss cheese
Slice your ciabatta bun, and spread the butter on both sides. Place the bun halves butter-side-down in the pan until lightly toasted, 2-3 minutes.
1 ciabatta roll, 1 tablespoon salted butter
Slather both sides of each slice of bread with the horseradish mayo sauce, then add arugula, the corned beef and cheese, and sliced pickles to one half of the bun.
6 dill pickle chips, ¼ cup fresh arugula
Place the top of the bun on, skewer with a toothpick if desired, and enjoy! Best served with an extra side of horseradish.