In a large steamer, steam the eggplant for 10 to 12 minutes, until very tender. Turn on the broiler.
Combine the miso and tamari in a small bowl. Make a 1/2-inch-deep lengthwise slit down the eggplants. Spoon 2 tablespoons of miso into each eggplant.
Place on a baking sheet and slide under the broiler until slightly browned, about 2 to 3 minutes.
We used white miso, and couldn't find Japanese eggplant, so used the longest smallest eggplants we could find -- they just necessitated steaming for several extra minutes. Overall, the dish was very salty from the miso/tamari mixture. I had to scrape most of it out, but the remaining eggplant had great flavor. Note that the miso burns quickly if you don't watch it carefully under the broiler.