Put all the ingredients for the broth in a pot and leave for 10 minutes. Bring to a boil and remove the kelp. Lower the heat, cover and simmer on low for 5 minutes. If you are using shiitake mushrooms for topping, add them to the broth and simmer together.
Bring a pot of water to boil and add your udon noodles. Cook noodles according to the directions on the package. Save some of the water (about a cup), drain and set aside.
Divide the noodles and broth between two bowls and add topping of your choice. Serve hot.
Add a little of the hot udon water that you saved to dilute the broth if it’s too strong.
You can refrigerate the broth in an airtight storage container. It will keep for up to 1 month.
Or you can freeze the broth in an airtight storage container (let it cool to room temperature first) and it will last for up to 3 months.