In a medium saucepan, combine the piece of kombu, optional ginger slices, and 2 quarts of cold water.
Place the pan over medium heat.
Warm the water gently, watching it closely.
Do not allow it to come to a rolling boil.
As soon as small bubbles begin to form around the edges of the pan, turn off the heat and immediately remove the kombu.
If you used ginger, allow it to steep in the hot water for another 5 minutes to infuse its flavor, then strain it out.
Use your fresh dashi immediately, or let it cool and store it in an airtight container in the refrigerator for up to 3 days.
Don't discard the used kombu!
Slice it thinly and add it to salads, stir-fries, or simmer it with soy sauce and mirin to make a delicious side dish called tsukudani.
