Heat the oven to 400° F (204° C). Line a baking tray with parchment paper.
Drain the tofu. Wrap in paper towels or a tea towel and place something heavy (like a cast iron pan) on top. Press for 15-20 minutes. Removing the excess water from the tofu will ensure that it gets extra crispy when it's baked.
Cut the tofu into 3 even slabs lengthwise. Stack the slabs on top of each other and slice them 3 times lengthwise and 5 times crosswise.
Transfer the tofu to a shallow baking dish. Add 1 tablespoon of vegetable oil and 1 tablespoon of soy sauce. Toss gently to coat. Let the tofu marinade while you prepare the salt and pepper dredge.
Combine the cornstarch, salt, white pepper and Chinese 5 spice in a large shallow bowl.
Dredge the tofu cubes in the cornstarch and spices and transfer the pieces to the prepared baking tray, leaving space between each piece. Bake for 30 minutes, turning the tofu over halfway through.
While the tofu is cooking, heat a dry cast iron pan over low heat. Add the black peppercorns and Sichuan peppercorns to the pan and roast for two minutes. Transfer to a mortar and pestle. Add the Chinese Five Spice and grind everything together.
In a non stick pan, heat the remaining tablespoon of vegetable oil. Add the onions, green and red peppers, and chili peppers to the pan. Saute for 6 minutes. At the 5 minute mark, turn the heat down and add the minced garlic.
Add the crispy baked tofu to the pan with the veggies, sprinkle with spices and toss to coat.