Heat a saucepan on the stove over medium heat, add the soy sauce, mirin, brown sugar, garlic and ginger to the pan, whisk to combine.
Bring to a boil, then simmer 4-5 minutes.
In a small bowl, combine the cornstarch and water.
Add this to the boiling sauce and whisk together, reduce the heat to low and simmer 3-4 minutes. Remove from the heat and set aside to cool.
Teriyaki Steak Shish Kabobs
Once cooled, place ½ cup of teriyaki sauce in a large bowl (reserve the remaining ½ cup of teriyaki sauce), whisk in the pineapple juice and crushed red pepper.
Add the sirloin.
Place in the refrigerator and marinate for at least 15 minutes, or up to 5 hours.
Remove the steak from the marinade and skewer with the bell peppers, onion and pineapple.
Heat one side of a grill over medium heat & another side over low heat.
Grill skewers over medium heat until beef is well seared on all sides, about 3 minutes per side.
Move skewers to the low heat side of the grill and brush all over with the reserved teriyaki sauce.
Cover grill and continue to cook 5-7 minutes.
Transfer skewers to platter and let rest for 5 minutes before serving.