Whisk the soy sauce, mirin, brown sugar, garlic and salt together in a large bowl then add the chicken and stir to combine.
Cover and refrigerate for at least 30 minutes or up to 6 hours to marinate.
Drain the chicken from the marinade then heat 1 tablespoon of the oil in a large skillet or wok pan.
Dredge 12-15 pieces of chicken in 1 cup of the cornstarch then shake off the excess and fry in the pan for 5-6 minutes, tossing every minute or so to prevent burning.
Remove the chicken then repeat the cooking process with the remaining chicken, adding another tablespoon of oil before each batch. (There should be about 3 batches in total.)
Heat another tablespoon of oil in the pan then add the bell pepper and stir and cook for 2-3 minutes or until they start to soften.
Add the pineapple and cook for another 3-4 minutes or until it starts to caramelize.
In the meantime, whisk the pineapple juice and remaining tablespoon of cornstarch together in a medium bowl then pour it into the pan.
Continue stirring until it starts to thicken then add the chicken back to the pan and toss to combine.
Serve with steamed white rice and freshly sliced green onions and sesame seeds for garnish if desired.
NOTE ON MIRIN: Mirin is usually pretty easy to find in the Asian foods section of your grocery store, or online. If necessary you can substitute it for rice wine or white wine vinegar.NOTE ON PINEAPPLE: You can use canned pineapple or frozen (thawed) if you don't have fresh.