Combine the ingredients for the oyster sauce and set aside.
Drain the tofu and pat it dry with kitchen paper before cutting it into 20 equal-sized cubes. Pour enough oil into the wok to deep-fry the tofu, and heat it to 190°C (375°F), or until a cube of bread dropped into the oil turns golden in 20 seconds. Add half the tofu and deep-fry until puffy and golden, then remove using a slotted spoon and drain on kitchen paper. Fry the rest of the tofu in the same way. Set aside.
Pour off all but 2 tablespoons of the oil and place the wok over a high heat. Add the ginger, garlic and spring onions and give them a quick stir, then add the water chestnuts, mangetout and red onion. Stir-fry for 2 minutes, until softened slightly, then add the sauce.
Stir-fry for 1 more minute, then return the tofu to the wok. Sprinkle the sesame oil over it and cook, stirring, for 1 minute for the tofu to absorb some of the flavours from the sauce. Serve immediately.