Trim the ends of eggplants and thinly slice into 1/4-inch slices. Arrange slices in a shallow dish, lightly salt and pepper, and toss with vegetable oil until coated. Set aside for a moment.
In a small bowl, whisk soy sauce, miso, vinegar, honey, sriracha (if using), garlic, and ginger until smooth.
Pour soy mixture over eggplant slices in dish and toss to evenly coat. Allow to sit for 10 minutes then toss slices and allow to sit another 10 minutes.
Grease the grill grates on an outdoor grill and preheat to 400F. Alternatively, lightly grease a large cast iron grill pan and heat on stovetop over medium-high heat.
Remove eggplant slices from soy mixture (do not discard marinade). Place eggplant slices in a single layer on preheated grill grates or work in batches and place eggplant in a single layer in preheated pan. Grill eggplant 2-3 minutes per side until eggplant is tender and lightly charred. Place eggplant on a serving plate.
Drizzle any remaining marinade over grilled eggplant and top with green onions for serving. Serve immediately and enjoy!