To make the sauce, whisk together all of the ingredients in a bowl or jug. Set aside.
Toss the tofu cubes with enough cornflour to lightly coat them. Heat 1 Tbsp of the oil in a wok over a medium heat and add the tofu. Cook, turning them regularly, until they are evenly browned then remove from the wok and set aside.
Heat the remaining Tbsp of oil in the wok and add the broccoli. Cook for about 5 minutes, stirring regularly, until the broccoli is just tender. Add the pepper and red onion and cook for a couple more minutes until slightly softened then add the pak choi and cook until wilted.
Add the sauce and stir through then stir in the tofu. Season to taste with salt and pepper then serve with rice or noodles.