In a large bowl, whisk together the brown sugar, soy sauce, chicken broth, apple cider vinegar, minced garlic, grated ginger, and red pepper flakes (if using). Add the chicken breasts to the marinade, ensuring they're fully coated. Cover and refrigerate for at least 30 minutes (or up to 4 hours for more flavor).
Heat olive oil in a large skillet over medium-high heat. If grilling, preheat your grill to medium-high.
Remove the chicken from the marinade, allowing excess to drip off. Reserve the marinade. If using a skillet, sear the chicken for about 6-7 minutes on each side or until the internal temperature reaches 165°F (75°C). If grilling, cook on each side for about 6-8 minutes.
While the chicken is cooking, pour the reserved marinade into a small saucepan. Bring it to a boil, then reduce to a simmer for about 5-7 minutes until it thickens slightly.
During the last couple of minutes of cooking the chicken, add the pineapple chunks to the skillet or grill. Let them caramelize lightly with the chicken.
Once the chicken is done, drizzle the thickened sauce over the chicken and pineapple. Let it rest for a couple of minutes before slicing the chicken into strips.
Serve the Brown Sugar Pineapple Chicken on a platter, garnished with chopped green onions.
Notes
For extra flavor, marinate the chicken for up to 4 hours.