Preheat the broiler to 550 degrees. Spread both sides of the buns with butter and place on a baking sheet. Broil for 2-3 minutes or until golden brown. Remove from the oven, then remove the top buns from the baking sheet and set aside.
Divide the shredded beef evenly between the four bottom buns. Top with Swiss cheese, then place back under the broiler for another 2-3 minutes, until melty.
Top the cheese with tomato, onion, and lettuce, then sprinkle with salt.
If using oil and vinegar: drizzle the lettuce with equal parts oil and vinegar, then sprinkle with oregano. Add the banana peppers on top, sandwich with the top bun, and enjoy!
If using horseradish mayo: Combine horseradish and mayonnaise in a bowl, then spread a thick layer on the top bun. Sandwich the lettuce and enjoy!