Prepare the chicken by cutting it into bite-sized pieces and seasoning with salt and pepper.
Heat sesame oil in a large skillet over medium-high heat.
Add the seasoned chicken to the skillet and cook until browned, about 5-7 minutes.
Add the chopped onion and garlic, cooking until softened, about 3-4 minutes.
Stir in the diced bell pepper and pineapple, cooking for an additional 3-4 minutes.
Pour in the soy sauce and bring the mixture to a simmer.
Mix the cornstarch slurry into the skillet, stirring until the sauce thickens.
Fold in the cooked rice, allowing everything to combine for 2-3 minutes.
Adjust seasoning as needed and serve hot, garnished with sesame seeds.