Prepare: Preheat the oven to 425 F and line a baking sheet with parchment paper.
Coat Tofu: Place the cubed tofu in a bowl with the oil, soy sauce, cornstarch, garlic powder and black pepper. Gently mix with a rubber spatula to coat.
Bake Tofu: Place the tofu cubes on the baking sheet and bake for 20-25 minutes until browned and puffy. Note: The tofu can also be pan-fried, see notes, though I prefer the texture of baked.
Mix Sauce: Once the tofu is in the oven, mix the sauce ingredients in any container or bowl.
Start Stir Fry: When the tofu has just under 10 minutes left, heat the sesame oil in a large wok or non-stick skillet over medium heat. Add the garlic and ginger and cook for 1-2 minutes, stirring frequently. Add the bell peppers and onions and cook for 3 minutes, stirring often, until tender but still crisp.
Add Sauce and Tofu: Stir the sauce again and pour it into the pan. Bring to a simmer and cook while stirring for 1-2 minutes until it starts thickening. Add the baked tofu cubes, mix well to coat in sauce and cook for another few minutes until thickened to your liking.
Serve: Serve the kung pao tofu over rice or with lettuce cups. Top with the crushed cashews and chopped green onions.