In a small bowl, whisk together the vegetable broth, soy sauce, cornstarch, hoisin sauce, vegan oyster sauce, and sugar.
Heat the oil in a large skillet or wok over medium-high heat.
Add the onion and garlic, and stir-fry for 1-2 minutes until fragrant.
Add the cabbage, broccoli florets, and red bell pepper to the skillet and stir-fry for 2-3 minutes. Then add carrots, mushrooms, baby corn, and water chestnuts, and stir-fry for another 2 minutes.
Add the tofu or seitan and stir-fry for 2-3 minutes until heated through.
Pour the sauce over the vegetables and tofu or seitan, and stir-fry for an additional 2-3 minutes until the sauce has thickened and coated the ingredients.
Serve the vegan chop suey over rice or noodles. Enjoy!