Put the stock cube into a pan of water, and then bring to boil. Add the noodles, bring the stock back to the boil and cook for two minutes. Add the broccoli and boil for two minutes more. Reserve a cup of the stock, then drain the noodles and veg. Heat a frying pan or wok, add the sesame oil and stir-fry the mushrooms for two minutes until turning golden. Add the garlic, chilli flakes and most of the spring onions, cook for one minute more, and then tip in the noodles and broccoli. Splash in three tablespoons of stock and the hoisin sauce, toss together for one minute. Serve the noodles scattered with the cashew nuts and remaining spring onions. Add a dash more sesame oil to taste, if you like.