In a medium-sized bowl, mix together the soy sauce, ketchup, honey, sesame oil, diced onion, minced garlic, and red pepper flakes. Pour the mixture over the chicken.
Cover and cook on Low for 3 to 4 hours or on High for 1 1/2 to 2 1/2 hours, until the chicken is cooked through (165 F). When done cooking, remove the chicken from the Crockpot, and place it in a bowl or on a cutting board to rest.
Stir together the cornstarch and water, and add to the liquid in the Crockpot. Turn the Crockpot back on High, and cook the sauce for 10 to 15 minutes, stirring occasionally, until it has thickened.
Shred the chicken, and add it back to the Crockpot. Add salt and pepper to taste.