Start by seasoning your chicken with red pepper flakes, garlic salt, onion powder, paprika, and black pepper.
Heat a large skillet over medium-high and sear the chicken for about 3 minutes per side—just enough to give it a bit of color (it’ll finish cooking in the slow cooker).
Transfer the chicken to the bottom of your slow cooker.
Add the chopped red bell pepper, red onion, and pineapple chunks right on top.
In a small bowl, whisk together the reserved 1 cup of pineapple juice, brown sugar, soy sauce, honey, vinegar, and cornstarch until smooth.
Pour that sweet and tangy sauce all over the chicken and veggies.
Pop the lid on and cook on low for 6 hours or high for 4 hours—until the chicken is tender and the flavors have come together.
Garnish with chopped scallions just before serving, and that’s it!