1Prepare Dashi: In a pot, combine 1000g water and 10g kombu. Let it soak for 30 minutes, then heat over medium-low heat. Just before it boils, remove the kombu. Bring the water to a boil, then turn off the heat and add 20g katsuobushi. Let it steep for 5 minutes, then strain the dashi through a fine-mesh sieve, discarding the katsuobushi.
3Cook Udon: Bring a large pot of water to a rolling boil. Add the 400g fresh udon noodles and cook according to package directions, typically 2-3 minutes for fresh udon, or until tender. Drain well and set aside.