Begin by taking a large saucepan and adding soy sauce, mirin, sake, kombu and katsuobushi to it. It is generally recommended to use dark soy sauce instead of light soy sauce as it helps provide the sauce with a richer flavour. Also, you can use shiitake mushrooms instead of katsuobushi if you want to prepare a vegan version of this sauce.
Adjust the heat to medium-high and wait till the stock comes to a boil.
Now, make sure to reduce the heat to a low as soon as it reaches a boil. Allow the soup to simmer for around 5–7 minutes. Keep in mind that men Tsuyu should thicken a little before you remove it from the heat.
Once done, take the pan off the heat and, using a fine mesh strainer, carefully strain the prepared sauce to get rid of the kombu and katsuobushi, and there you have it!
Allow the prepared sauce to cool completely before adding it to other dishes or storing it for future usage. The men Tsuyu will be fit for consumption for about a week. And remember to store it in an airtight container and place it in the refrigerator.