This is the knife you need:
The correct technique is to face away from the cutting board, throw the sandwich in an arc over your shoulder, then spin and bisect the sandwich in midair before it lands on the plate.
The whole point of mustard or mayo is to hold the sandwich together in the air (the Japanese word for mayo translates literally to "warrior glue").
[photo by Rich Legg, http://leggnet.com/2013/12/hands-of-a-maguro-bocho-master.html]