Prepare outdoor grill for direct grilling on medium. Sprinkle eggplant slices with ½ teaspoon salt. Arrange between layers of paper towel; let stand 15 minutes.
Meanwhile, stack two 24-inch-long sheets aluminum foil. Place beans on half of foil; toss with 2 teaspoons oil. Fold other half of foil over; crimp edges to seal. Grill 16 minutes, turning over once. Set aside.
Cook soba as label directs. Drain and rinse with cold water. Drain again; set aside.
In large bowl, whisk miso, sugar, vinegar, soy sauce, garlic, ginger, remaining 3 tablespoons sesame oil, and 2 tablespoons water. Firmly press liquid from eggplants with dry paper towels. Brush eggplants with some miso mixture (reserve remaining). Grill eggplants, covered, 8 to 10 minutes or until just tender, turning over once. Cut into bite-size pieces.
Toss soba, green beans, and eggplants with miso in bowl. Serve on spinach; top with onions and sesame.
