Butter each slice of bread on one side. Flip over and spread mustard on both pieces of bread.
Cut the knockwurst almost the way in half lengthwise, so it opens like a book.
Heat a cast iron over medium heat. Once hot, add knockwurst, cut end down, until browned, about 3 minutes. Flip and grill the other side until the casing blisters slightly. Remove from heat.
Place pile of pastrami on the bread. Top with Swiss cheese, the grilled knockwurst, and final slice of bread.
Lower the heat slightly. Carefully place sandwich on the hot cast iron and grill until side is brown, about 3 - 4 minutes. Flip and cover with a lid or sheet pan. (This will help the cheese melt). Remove when other side is browned.