Whisk ketchup, brown sugar, soy sauce, vinegar, broth, pineapple juice, garlic, ginger, and red pepper flakes.
Season chicken with salt and pepper. Spread onions in the slow cooker and place chicken on top.
Pour the sauce over the chicken. Cover and cook on LOW for 5 hours, or HIGH for 2.5–3 hours.
Stir cornstarch with cold water. Switch to HIGH, stir the slurry into the sauce, and cook 10–15 minutes until thick.
Serve the chicken with sauce over rice. Garnish with green onions and sesame seeds.
Notes
Swap chicken thighs for breasts if you prefer leaner meat; reduce cook time by about 30 minutes. For extra tang, add 1–2 tsp more cider vinegar at the end. Leftovers keep 4 days refrigerated and reheat well; thin with a splash of broth if needed.This recipe is an original creation inspired by classic Slow Cooker Sweet and Tangy Chicken Recipes flavors. All ingredient ratios and instructions are independently developed.