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.