Mix together the sauce ingredients in a small bowl. Set aside.
Cook your udon according to package instructions, drain and rinse with cold water. Set aside.
Heat 2 tablespoons of vegetable oil in a stir fry pan, or wok, over a high heat.
Once the oil is very hot, add in your onions and mushrooms. Fry, stirring or tossing constantly, until the onions have browned and are almost crispy, around 2-3 minutes.
Now add in your cabbage, spring onions and garlic. Fry, stirring or tossing constantly, until the cabbage is starting to brown but is still slightly crunchy, around 2-3 minutes.
Add in the udon noodles and stir fry for 2 minutes.
Add in your sauce and stir fry for 1 more minute.
Serve topped with the extra spring onion and sesame seeds.