Combine everything except tofu in a pot. Bring to a boil, then lower the heat to a simmer. Simmer for 45 minutes to 1 hour.
Stir in tofu to heat through.
Ladle into individual serving bowls, garnish with chopped spring onion and serve with rice.
NOTES
- If you don't have mirin, substitute with sake and a pinch of caster sugar. - I used homemade vegetable stock (from boiling vegetables for mash). If using very salty canned broth, dilute the broth before using and/or reduce the amount of soy sauce. - Adapted from Cook for Your Life