In a small pan, whisk the miso, mirin, tamari and sugar. Place over medium heat and whisk as the mixture comes to a boil. Cook, stirring, to thicken slightly. Let cool.
Trim stems from the eggplants and halve each lengthwise. Cut slashes in the flesh of each half, almost to the skin but not through. Brush lightly with oil. Preheat grill. Place the eggplants on the hot grill, cut side down. Cook for 3 minutes, until the eggplant is marked and softened. Flip pieces cut side up, and drizzle with miso sauce, brushing the sauce to the edges. Grill another 3 minutes, until eggplant is tender and sauce bubbling. Serve hot.
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