150gonion, frenched (halved pole to pole, then thinly sliced)
150gcabbage, sliced into similar-sized pieces to the onion
700gwater (enough to cover vegetables; total soup weight (water + veg) ~1000g)
33gdashi stock concentrate (~10% salt by weight; scale so that dashi + soy deliver 0.5% salt of total soup weight, with dashi providing 2/3)
17gsoy sauce (~10% salt by weight; provides remaining 1/3 of pre-miso salt)
45gmiso paste (~10% salt by weight; added off heat — brings total soup salt to ~1% by weight)
Steps
Combine water, dashi concentrate, soy sauce, onion, and cabbage in a pot. Using boiling water from the kettle speeds this up. Bring to a boil, then simmer until vegetables are softened, about 10–15 minutes.
Remove from heat. Whisk in miso until fully dissolved. Do not boil after adding miso.
Serve immediately.
Notes
Additions:
Tofu — ~280g extra firm
Optionally pan-fry slices until golden before adding
Add raw tofu ~5 minutes before done
Add fried tofu ~2 minutes before done
Chicken thighs — add early, they'll be fine
Gyoza / store-bought dumplings — steam at the end for 6 minutes or per package instructions
Many other additions are possible — the broth is forgiving