Place the chicken thighs or breasts in the bottom of the slow cooker. Spread them out evenly so they cook at the same rate and soak up the sauce well as they cook.
In a medium bowl, whisk together the sweet chili sauce, soy sauce, rice vinegar, honey, minced garlic, and grated ginger. Stir until the mixture is smooth and fully combined. This sauce is the flavor base for the whole dish, giving it sweetness, tang, and savory depth.
Slice the red bell pepper, yellow bell pepper, and onion into even strips. Layer the vegetables right over the chicken in the slow cooker so they soften as they cook and add flavor to the sauce.
Pour the sauce mixture over the chicken and vegetables, making sure everything is coated as evenly as possible. Cover the slow cooker with the lid and cook on low for about 6 hours or on high for about 4 hours, until the chicken is tender and fully cooked through.
When the chicken is nearly done, mix the cornstarch and water together in a small bowl until smooth. This will help thicken the sauce without making it lumpy.
Stir the cornstarch slurry into the slow cooker. Cover again and cook on high for another 30 minutes, or until the sauce thickens and becomes glossy enough to coat the chicken and vegetables nicely.
Once the sauce has thickened, give everything a gentle stir. If desired, you can shred the chicken slightly or leave it whole depending on how you want to serve it.
Serve the sweet chili chicken hot over cooked rice. Garnish with chopped green onions and sesame seeds for extra flavor, texture, and a clean finished look.