Cook the noodles: Blanch udon noodles in boiling water for about 1 to 2 minutes or if using dry noodles, cook according to package instructions. Drain well and immediately rinse with cold water. Set aside.
Make the sauce: Add all the sauce ingredients together to a bowl and whisk until combined.
Saute the sauteables: Heat the olive oil in a large skillet or a wok. Add the chicken pieces and cook until the chicken is no longer pink and starts to brown slightly. Add the mushrooms and cook for a couple more minutes until mushrooms are starting to brown.
Finish the dish: Add the udon noodles and prepared sauce to the wok, toss everything together well, and cook until the noodles are heated through. Garnish with green onions and serve hot.
