To the basin of a 6 quart slow cooker, add the cubed chicken, chopped onion, and sliced red bell pepper. In a medium bowl, whisk together the honey, soy sauce, ketchup, sesame oil and minced garlic until combined. Pour the sauce over the chicken and veggies and stir to combine. Cover and cook on LOW for 6 hours or HIGH for 4 hours. Once the cook time is up, in a small bowl, whisk together the cornstarch and water. Pour the mixture into the sauce and stir to distribute. This thickens the sauce. Before serving, toss in the cooked and drained spaghetti. Garnish with scallions and serve!