This filling vegetarian dish gets Asian flavor from rice vinegar, soy sauce, and ginger.
Fill a large pot with about 1 inch water; bring to a boil. Add eggplant and mushrooms. Reduce heat to medium-high; steam until tender, 8 to 10 minutes.
Meanwhile, place peanut butter in a small bowl. Gradually whisk in vinegar, then soy sauce, ginger, brown sugar, and 1 teaspoon salt until smooth.
With a slotted spoon, transfer steamed eggplant and mushrooms to a large bowl. Add scallions and peanut sauce; toss gently to coat. Serve with rice, if desired.