Fry the tofu: Cut the tofu into four thick slabs and pat dry. Season lightly with salt and coat with cornstarch. Fry in a thin layer of oil for 3 to 4 minutes per side, until golden. Drain, cut into bite-sized pieces, and set aside.
Make the sauce: Mix the soy sauce, oyster sauce, bouillon, water, cornstarch, sugar, and sesame oil. Set aside.
Blanch the firm vegetables: Bring a pot of water to a boil. Add a pinch of baking soda, if using. Blanch the cauliflower, carrots, broccolini, and white cabbage pieces for 1 to 2 minutes, until slightly tender but still crisp. Drain well.
Cook the aromatics: Heat about 1 tablespoon oil in a wok over medium heat. Cook the ginger for 30 seconds. Add the onion and garlic and cook until softened.
Stir-fry the vegetables: Raise the heat to medium-high. Add the mushrooms, baby corn, celery, and bell pepper. Cook for 1 to 2 minutes. Add the blanched vegetables and cook for another 2 minutes. Add the green cabbage leaves and cook just until wilted.
Add the sauce: Stir the sauce, then pour it into the pan. Toss with the vegetables and simmer for 1 to 2 minutes, until slightly thickened.
Add the tofu: Add the tofu and gently toss until coated. Taste and adjust. Serve hot with rice.