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.