But actually, it was easier than I thought.
I think many people make the broth themselves, either from konbu seaweed or using miso as a base, but for those of us who have no idea of how to make a decent broth, there are instant soup bases that can serve as the base for the nabe hotpot.
The flavor I chose was "tan-tan goma" or spicy broth with sesame. It's based on a Chinese soup recipe from some part of China, quite spicy. While some people buy a lot of different kinds of vegetables to put in, I held off on variety given that vegetables at this time of year are unsually expensive. I bought sliced pork (shoulder, very tasty), mushrooms, tofu, green onions and bakchoy cabbage.
The package in the photo actually serves up to 4, but I chose to use only 1/2 for each sitting. Even then the concentrated broth held up fine against the vegetables and tofu, whose water content leeches out during the cooking process. After stewing the whole mix for about 5 minutes I turned off the burner and dug in. How delicious!
From others' suggestions, I reserved some of the broth for the next day's breakfast. I reheated rice in the microwave and the leftover soup in a saucepan, then threw the reheated rice into the soup. Not quite exactly "zosui" style porridge, but close enough and a great way to beat the cold wave that's rolled over Japan. (It was probably subzero in Tokyo last night!)