In a large bowl, combine onion, soy sauce, honey, ketchup, vegetable oil, garlic, ginger and red pepper.
Season chicken thighs with salt and pepper, to taste. Place chicken thighs into a 6-qt slow cooker. Add soy sauce mixture and gently toss to combine. Cover and cook on low heat for 3 hours and 30 minutes.
Remove chicken thighs from the slow cooker and shred the chicken before returning to the pot with the juices. Cover and keep warm for an additional 30 minutes.
Serve immediately, garnished with green onions and sesame seeds, if desired.