Heat the oil in a large wok or skillet over medium-high heat.
Add the tofu to the pan and season with salt and pepper. Cook for about 5-7 minutes or until lightly browned.
Remove the tofu from the pan and set it aside.
Add additional oil if pan is too dry after tofu is removed
In the same pan, add the sliced onions and garlic. Cook for 2-3 minutes or until the onions are translucent.
Add the sliced bell peppers, broccoli florets, and water chestnuts to the pan. Cook for an additional 5-7 minutes or until the vegetables are tender but still slightly crispy.
Mix the cornstarch, oyster sauce, sesame oil, and Chinese five spice together in a small bowl.
Pour the sauce into the pan with the vegetables and stir to coat the vegetables evenly.
Add the tofu back into the pan and toss everything together until the tofu is coated in the sauce.
Sprinkle sesame seeds on top of the stir-fry and serve with rice or noodles