Melt butter in a large skillet over medium high heat. Add chicken, skin-side down, and sear both sides until golden brown, about 2-3 minutes per side; drain excess fat and set chicken aside.
In a large bowl, combine pineapple juice, chicken broth, brown sugar, soy sauce, honey, vinegar, Sriracha, garlic and ginger.
Place chicken thighs into a 6-qt slow cooker. Stir in pineapple juice mixture. Top with pineapple chunks, crushed pineapple and reserved juices.
Cover and cook on low heat for 5-6 hours or high for 2-3 hours, basting every few hours.
In a small bowl, whisk together 1/4 cup water and cornstarch. Stir in cornstarch mixture, bell pepper and onion into the slow cooker. Cover and cook on high heat for an additional 30-60 minutes, or until sauce is thickened.
Serve chicken immediately, garnished with sesame seeds and parsley, if desired.