Talk to the plumber. Pour him a normal swill by colleting the tankard and filling it from the swill tap.
He likes mushroom swill. There is an ad about mushrooms in the wall. Take it.
There were mushrooms in the pipe forest. Maybe the witch knows about mushrooms.
Give the ad to the witch. You get three guesses.
As one of you guesses, talk to the witch.
Mix the mushroom into a tankard full of swill. Serve it to the plumber. Two a-zlottis, ka-ching!