Combine all seasoning ingredients in a pot and heat until boiling. Crumble the oboro tofu into the pot and simmer over medium heat until warmed through.

Turn off the heat and add the cornstarch slurry (cornstarch mixed with water). Stir continuously over medium heat until the soup thickens.

Slowly pour the beaten egg into the soup in a thin stream. Once it starts to set, gently stir and cook through.