In a large pan, dissolve the stock cube in 1 litre of water and stir in the teriyaki sauce. While the soup base comes to the boil, heat the oil in a frying pan and cook the mushrooms over a high heat, for 2-3 mins, until they turn golden. Add the spring onions and cook for 1 min more, then set aside.
Once the soup base has come to the boil, add the noodles and cook for 4 mins. Add the spinach and cook for 1 min more until just wilted. Stir in the mushrooms, spring onions and some seasoning, and serve.