Season the diced chicken with salt and pepper.
Heat vegetable oil in a large skillet over medium-high heat.
Add the chicken and cook until browned, then remove from the skillet and set aside.
In the same skillet, sauté the onion and garlic until fragrant.
Add the red bell pepper and pineapple chunks, and cook until slightly softened.
In a bowl, mix ketchup, soy sauce, rice vinegar, brown sugar, and cornstarch.
Pour the sauce into the skillet and bring to a simmer.
Add the cooked chicken back into the skillet and stir to coat in the sauce.
Simmer for a few more minutes until the sauce thickens.
Adjust seasoning with salt and pepper to taste.
Serve the Tropical Chicken with Pineapple over rice and garnish with sesame seeds and green onions.