Season the chicken chunks lightly with salt and pepper and put them in a slow cooker. Sprinkle over the five-spice powder. Stir together the stock, soy sauce, ginger, sriracha and garlic in a small bowl and pour over the chicken. Cover and cook on high for 3 1/2 hours.
After 3 1/2 hours, mix together the cornstarch with 2 tablespoons water in a small bowl. Stir it into the slow cooker along with the sesame oil. Add the broccoli and peppers, cover and continue cooking on high for 30 minutes. Taste the sauce and adjust the seasoning.
Serve over rice garnished with scallions and sesame seeds.