Generously coat the bottom of a medium nonstick skillet with vegetable oil and place it over medium heat. Get the oil to at least ⅛-inch deep. You want the tofu pieces to be about halfway submerged when you add them.
While the oil heats up, stir the cornstarch, black pepper, white pepper, salt, and five-spice together in a small bowl. Add a few pieces of tofu to the bowl and dredge them in the mixture.
Once the oil begins to shimmer, remove the tofu cubes from the bowl, shake off any excess breading, then add them to the skillet. Repeat this process, placing the tofu cubes in a single layer so they are not touching (Note 1). Fry the tofu pieces for 4 to 5 minutes on each side, until golden and crispy. Transfer them to a plate when done.
If you still have a lot of oil in the skillet, very carefully pour some out (Note 2), so you have just a thin coating. If the skillet is dry, you can add about a tablespoon. Raise the heat to medium-high, then add the onion, jalapeño, and Fresno peppers. Stir-fry the onion and peppers for about 4 minutes, stirring constantly, until they begin to soften and char (Note 3).
Push the onions and peppers to the side, then add the sliced garlic and white parts of your scallions to the skillet. Cook the garlic and scallions for 1 to 2 minutes, until they begin to brown and the garlic becomes very fragrant.
Return the tofu to the skillet. Stir everything up and cook the mixture for another 1 to 2 minutes, just to warm the tofu back up a bit.
Remove the skillet from heat, then stir in the green parts of your scallions. Serve.