Crack the egg into a bowl and mix well.

Combine all sauce ingredients (mirin, soy sauce, dashi, cornstarch, and water) in a bowl and mix thoroughly.

Trim the roots of the seri and cut into bite-sized pieces.

Bring water to a boil in a pot, add the tofu, and simmer gently over medium heat until warmed. Remove and drain excess water.

Add the sauce mixture to the pot and heat while stirring until it thickens. Add seri and egg, lightly mix, and simmer on low heat until the egg sets.