Mix the mustard, honey, orange zest and juice and 2 tablespoons olive oil in a large bowl. Season, add the chicken and stir to coat evenly. Cover and set aside for at least 15 minutes, or chill overnight.
This works equally well with strips of beef, pork or turkey, and can be served with rice instead of noodles, if you like.
Meanwhile, cook the noodles in boiling water according to the pack instructions. Drain well, tip into a large bowl and mix with the remaining oil to stop the noodles sticking together. Set aside, covered, to keep hot.
Put a large frying pan or wok over a high heat. When hot, add the cashews and cook for 2 minutes, stirring, until lightly toasted. Remove the nuts from the pan, set aside to cool, then roughly chop. Using a slotted spoon, lift the chicken from the marinade and add to the hot pan. Cook for 5 minutes, stirring, until golden all over.
Add the chicken marinade to the pan and bubble away for 1 minute. Gradually add the noodles, separating them with your fingers as you go, then add the tomatoes and most of the spring onions. Season and mix everything together to coat in the marinade. Stir-fry for 2 minutes, until piping hot and the vegetables have just started to wilt. Divide between bowls and scatter with the chopped cashews and remaining spring onions to serve.