Slice knockwurst into long slices.
Melt butter on a large griddle over medium heat. Throw corned beef onto the griddle and get it sizzling. Add knockwurst slices; cook 4 to 5 minutes.
Add bread, making sure it gets a good coat of butter and grease from the griddle. Cook for 1 minute, then flip. Place Swiss cheese on bread slices. Cover with a lid or pot to steam until cheese melts, about 1 minute.
Remove everything from the griddle and assemble sandwich. Place one slice of bread on a plate with the melted cheese facing up. Pile on corned beef, knockwurst slices, and mustard. Top with second slice of bread, cheese facing down. Eat hot.
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