Add the 2½ pounds chicken thighs to the slow cooker. Drain the pineapple chunks, reserving the juice, and set the chunks aside for later.
In a bowl, whisk together the reserved pineapple juice, ½ cup soy sauce, ⅓ cup brown sugar, ¼ cup ketchup, 3 cloves minced garlic, 1 tablespoon grated ginger, and ½ teaspoon red pepper flakes. Pour it over the chicken.
Cover and cook on low for 4 to 5 hours, or on high for 2 to 3 hours, until the chicken is tender and cooked through. Thighs are forgiving here, so they stay juicy even if they go a little long.
Add the pineapple chunks and the red bell pepper to the slow cooker. Stir the 3 tablespoons cornstarch into the 3 tablespoons water until smooth, then stir it in as well. This is what turns the thin cooking liquid into a glossy glaze that coats every piece.
Cover and cook on high for another 20 to 30 minutes, until the sauce has thickened and the peppers are crisp tender. Adding the pineapple and pepper at the end keeps the pineapple from dissolving and the peppers from going mushy, so you get real texture and color.
Serve over the 3 cups warm rice, spooning plenty of sauce over the top, and finish with the sliced green onions and sesame seeds.