Place the kombu and shiitake mushrooms in a pot with the water. Allow it to rest for one hour.

After the hour has passed, place the pot over medium heat. Bring to a boil. Right before the water comes to a boil, remove the kombu. Add the bonito flakes and simmer for fifteen minutes. Remove the pot from the heat and let it sit for five minutes. Strain through a mesh strainer into another container. You should have about ¾ cup of dashi stock, which can be stored in an airtight container in the refrigerator for up to five days.

Begin by placing all the beef in the freezer for at least an hour before you begin to make the beef bowls. This will make it much easier to cut the beef thinly. Once slightly chilled, carefully cut the meat as thin as possible.

Combine the dashi stock, soy sauce, sake, mirin, sugar, and garlic in a medium pot over medium heat and bring to a boil. Reduce the heat to low and allow the sauce to simmer lightly for ten minutes.

Return the heat to medium and add the onions. Cook the onions for about ten to fifteen minutes or until they are soft and translucent.

Add the ribeye. Simmer for five to eight minutes or until the meat is just cooked. To serve, place a ½ cup of cooked rice in a bowl and top with the simmered beef and onions. Pour a small amount of the sauce over the beef. Serve with pickled ginger.