Brown the tofu: Heat 1 tablespoon vegetable oil in a nonstick skillet or wok over medium-high heat. When the oil shimmers, crumble in the tofu and stir fry until golden brown. This usually takes around 8-10 minutes depending on how well you pressed your tofu. Once tofu is browned to your preference, remove it to a bowl and set aside.
Stir-fry aromatics: In the same pan/wok, heat another tablespoon vegetable oil over medium-high heat. Once oil is hot, add in the diced onion, white parts of the scallions, minced garlic, minced ginger, and chili paste (if using). Stir-fry 3-5 minutes, or until onions are translucent.
Add tofu and sauce: Add back in the browned tofu, along with the diced water chestnuts, soy sauce, and vegan oyster sauce (or hoisin sauce). Stir-fry for 3-5 minutes to distribute the sauces and to heat through. The sugars in the sauce may begin to caramelize slightly (this is good), but keep an eye on it and reduce the heat as needed so they don't burn.
Finishing touches: Taste and adjust to preference. Stir in the green parts of the scallions and serve.