Heat the peanut oil in a wok or large non-stick skillet over medium-high heat. Add the chicken chunks to the pan and cook until lightly browned and chicken is mostly cooked through, about 5 minutes. Add the bell pepper to the chicken and cook 3 more minutes. Transfer the chicken and pepper and any juices to a bowl and set aside.
Reduce the heat to medium-high heat and add the 1 cup water, soy sauce, sriracha, garlic, and ginger to the now empty pan and stir to combine.
In a separate small bowl whisk together the cornstarch and 1 TBSP water until cornstarch is fully dissolved. Stir the cornstarch mixture into the pan with the sauce. Cook until sauce is thickened, about 2 minutes.
Add the chicken and bell pepper to the sauce along with the pineapple, sesame oil and peanuts. Stir to combine and cook until heated through, about 2 minutes.
Transfer to a large serving dish and sprinkle with the sliced green onion. Serve with steamed rice.