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.
Notes
Leftover broth:
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.