In a mixing bowl, combine the brown sugar, soy sauce, minced garlic, grated ginger, sesame oil, black pepper, and red pepper flakes. Whisk until the brown sugar is dissolved to create a marinade.
Place the chicken breasts in a resealable plastic bag or a shallow dish, then pour the marinade over the chicken. Seal the bag or cover the dish and refrigerate for at least 1 hour, preferably 3 hours.
Preheat your grill or stovetop grill pan over medium-high heat.
Remove the chicken from the marinade and allow excess liquid to drip off. Discard the remaining marinade.
Grill the chicken for about 6-7 minutes on each side, or until the internal temperature reaches 165°F (75°C) and the chicken is nicely browned. During the last 2 minutes of cooking, add the diced pineapple to the grill to caramelize slightly.
Once cooked, remove the chicken and pineapple from the grill and let it rest for 5 minutes.
Slice the chicken and serve it topped with the grilled pineapple. Garnish with chopped green onions for an added layer of flavor and color.
Notes
For best results, marinate the chicken for 3 hours.