Mix all the ingredients for the sauce with a hand mixer until smooth. Set aside.
Boil salted water in a big pot. Cook the udon noodles according to the packet instructions but remove them 30 seconds before. Drain the water and add a little oil so they don't stick together.
Cut up the broccoli. Rinse and cut up the pak choi and peppers.
Heat a wok pan with the oil. Add the broccoli and stir-fry for 2 minutes.
Add the thicker white part of the pak choi, peppers and pieces of tofu and fry for a further 1 min. Add the rest of the pak choi and fry for a further 2 mins, stirring.
Pour the contents of the wok into a large plate or bowl.
Pour the sauce into the wok and let it bubble up before adding the noodles and stirring. When all the noodles are evenly covered, add the vegetables and tofu and stir.
Serve with chopped peanuts and maybe some coriander if you like that sort of thing.