Place the water, soy sauce, mirin, sesame oil and caster sugar in a casserole dish. Bring to a simmer over a moderate heat. Reduce the heat and add the beef, allowing it to poach gently for 6-8 minutes until rare.
Add the noodles, misome (or spinach), mushrooms, and tofu towards the end of cooking to heat through gently. Spoon into serving bowls, making sure not to add too much of the broth. Serve immediately.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.