Step 1

In a large pot, combine 8 cups water and kombu. Allow to sit for 30 minutes. After 30 minutes, bring to a boil over medium heat. Remove the kombu pieces and add remaining 2 tablespoons cold water. Stir in the bonito flakes until just combined. Once boiling, reduce to low heat and simmer for 5 minutes. Skim off any foam that develops on the surface. Remove from heat and allow to sit for 15 minutes. Pour through a fine mesh strainer or cheesecloth into a large bowl. Don't squeeze excess liquid from bonito and discard.

Step 2

In a medium bowl, whisk together sake, sugar, 1/4 cup soy sauce, and grated ginger. Add the shaved beef pieces, tossing to coat well. Allow to marinade for 10 minutes.

Step 3

Place a large skillet over medium high heat. Do not add oil. Once heated, add the beef with marinate, spreading out to cover the entire base of pan. Cook, stirring often, until most of marinade has evaporated and meat is browned, about 5 minutes.

Step 4

Return strained dashi (water cooked with kombu and bonito) to cleaned large pot over high heat. Stir in the mirin, 1/2 cup usukuchi soy sauce, and salt. Once boiling, reduce heat to low until ready to serve.

Step 5

In another large pot, bring water to a boil over high heat. Add frozen udon and cook until just tender. Drain and divide the udon among 4 large soup bowls. Pour the hot broth over the noodles and top each bowl with cooked meat. Top with sliced scallion and shichimi togarashi.

Step 6

Serve immediately.