Mix all the sauce ingredients in a bowl until combined.
In your medium pan, boil about 1.5 liters of water. Add the udon noodles and cook for about 1 minute. Carefully separate the noodles in the bowl using a chopstick or a fork. Strain and run in cold water to stop the cooking process. (Note that these packets are already cooked, so you want to avoid overcooking or they will be soft and mushy)
In the same pan, heat it medium high heat and add 1 tbsp cooking oil. Sear the sliced chicken on at least 2 sides. Add salt for seasoning and fry until cooked through. Set aside. (This is to prevent your chicken from overcooking so your chicken breast don't dry out)
In the same pan without washing, heat your pan to medium high heat. Add the rest of the cooking oil (1tbsp) and stir fry your onions until translucent and aromatic.
Add your mushrooms to the pan and stir fry until cooked through.
Put the sauce to the pan and heat until sugar is dissolved.
Put back the chicken that was set aside, as well as the leek and zucchini. Stir until sauce is evenly distributed to all the ingredients and let cook for about 1 minute.
Add your strained noodles to the pan and mix until sauce is evenly distributed.
Serve immediately.