Cut all the pork belly slices in half (crosswise). Set aside.
Place one pork belly slice on a plate. Place one shiso leaf on top of the pork.
Roll it up tightly (if the rolls are loose it will come apart when cooking). Repeat until you've used all the pork belly and shiso. There should be 20 pieces.
In a small bowl, mix together the soy sauce, mirin, and sake. Set aside.
In a large pan over high heat, pan fry the rolled pork belly and shiso pieces until they are nicely browned on all sides. This should just take about 5 minutes.
Once browned, turn the heat to low and pour the sauce mixture into the pan. Let the sauce cook down and lightly coat the rolls.
Turn off the heat. Remove the rolls onto a plate. Top with chopped green onions and sesame seeds (optional). Drizzle on any extra sauce left in the pan. Eat hot and enjoy ^_^