Sauce: In a bowl, combine all the ingredients. Set aside.
Stir-Fry: In a large bowl, dust the tofu with the cornstarch. Shake to remove any excess. Set aside on a plate.
In a wok, heat the oil. Fry the tofu, half at a time, until lightly browned. Season with salt and pepper. Remove the tofu from the wok with a slotted spoon and set aside on a plate lined with paper towels.
In the wok, fry the onions, ginger, and garlic for 1 to 2 minutes. Add oil, if needed. Add the sauce and bring to a boil. Add the tofu and cook, stirring to warm through, and coat thoroughly with the sauce. Serve immediately over rice. Serve with broccoli.