In a large bowl, combine the chicken pieces, Chinese five spice powder, garlic powder, onion powder, salt, and black pepper. Mix well to coat the chicken evenly. Let it marinate for at least 15 minutes for better flavor.
Heat the vegetable oil in a large skillet or wok over medium-high heat. Add the marinated chicken and stir-fry for about 5-7 minutes until the chicken is cooked through and lightly browned.
Add the sliced bell peppers, broccoli florets, and snap peas to the skillet. Stir-fry for another 3-5 minutes until the vegetables are tender-crisp.
Pour in the soy sauce and sesame oil, stirring to combine. If you prefer a thicker sauce, add the cornstarch mixture and cook for an additional minute until the sauce thickens.
Remove from heat and stir in the chopped green onions. Serve hot over rice or noodles.