Place the onion in a microwavable bowl. Cover and microwave on high for 30 seconds. Stir then repeat for 30 seconds.
Remove. Add the mushrooms, soy sauce and miso paste and 100ml vegetable stock. Stir well and break down the miso paste with the back of a spoon until well combined.
Cover and heat for one minute. Stir and heat for another minute. If you've used dried noodles you might need to cook for longer.