A Japanese stock recipe. You can make two broths from the seaweed and the tuna. The first broth made from them is called "ichiban dashi"; the second one is called "niban dashi". Hon Dashi is often used for clear soups in Japanese cooking.
Use a piece of kombu seaweed about 6 inches (15 cm) long.
Wipe any dirt off the seaweed, but don't try to wipe off the white powder on it: the powder adds flavour to the stock. Put water and the seaweed in a pot, and let soak for up to two hours.
Over low heat, bring the water to the point where it is just about to start boiling. Remove the kombu and set aside. Add the tuna flakes, and let the water come to a boil. As soon as it does, remove the pan from the heat.
When the tuna flakes have all settled to bottom, strain the stock to remove the tuna flakes.
If you are going to make Secondary Dashi ("niban dashi"), then reserve both the used seaweed and the used tuna flakes. Otherwise, you can discard them.