Soak a piece of kombu (12x5cm) for 30 minutes in the water and rehydrate the mushrooms in hot water.
Meanwhile, cut the tofu into bite-size pieces. Set aside.
Bring the kombu water to a boil. When bubbles start appearing, take out the seaweed, and add soy sauce, mirin and sugar. Set aside.
Cook noodles according to instruction, and serve immediately in bowls, pouring broth on top and decorating with the tofu, mushrooms (here sprinkled with shichimi togarashi) and greens (here my spinach salad – see recipe).