Bring the dashi stock almost to a boil in a saucepan and add the sliced mushrooms. Reduce the heat and simmer the mushrooms for 1 minute, then add the tofu. Reduce the heat to very low.
Place the miso in a small bowl and ladle some of the hot dashi stock over it. Stir with a wooden spoon until the miso is well dissolved. Pour the dissolved miso in