Made a well with the dry ingredients, and put the wet ingredients in the middle. Knead vigorously, and adjust the amount of water you use until you get a smooth, soft but firm dough. The dough should have a similar feel to your earlobe. Knead for at least 15 minutes. Cover with plastic wrap and leave for 2-3 hours for the gluten to activate.
Roll out the dough on a floured surface, and slice into thin strips with a sharp knife. I put a flat straight surface on top (such as a piece of cardboard) as a guide and used a pizza roller.
Put a sieve in a pot of rapidly boiling salted water and cook in batches (inside the sieve), agitating the sieve to keep the water moving and the noodles from sticking, for about 3-4 minutes, until al-dente. Rinse under cold water, and set aside.
If using a broth - Combine all the ingredients and bring to a simmer - avoid boiling the broth.If using a dipping sauce - bring to s simmer all the ingredients except the bonito flakes - add once the sauce is simmering, stir through, leave for 10 seconds and then strain out and discard the bonito flakes. You can assemble the noodles in a bowl, pour the broth over and top with the toppings, or, as shown in the photo, arrange noodles, nori, spring onion, sprouts and sichimi on a plate, with dipping sauce alongside.