Refrigerate your flatiron steak until you're ready to use it.
Using a very sharp long knife, slice the flatiron steak against the grain into thin strips (about 1/4 inch thick).
In a bowl or container, make the marinade for the teriyaki beef jerky.
Combine the soy sauce, mirin and sugar and whisk until the sugar dissolves.
Add the garlic junkie, crushed garlic cloves and the ginger.
Whisk until well combined.
Adjust the flavors to taste.
Add the sliced meat to the marinade and turn to coat.
Cover the container and refrigerate for at least 12 hours-- and up to 2 days.
Preheat your Traeger Grill to 225* with the super smoke feature enabled.
Remove the meat from the fridge.
Prepare two trays for smoking-- line the cookie sheet with aluminum foil for easy cleanup.
Place a wire rack atop the aluminum foil.
Arrange the marinated meat slices on the wire rack, leaving room between pieces.
Place the trays in the preheated Traeger grill and close the lid.
Allow the meat to smoke about 90 minutes before checking.
If necessary, rotate the trays for even cooking.
Flip over pieces that are thicker or drying less evenly.
Continue smoking the jerky slices for another 2 hours, until dry and shiny.
Remove from the grill and allow to cool to room temperature.
Store the slices in an airtight container in the fridge.
Enjoy your Traeger smoked teriyaki beef jerky within a week or so.