Place vegetable oil, sesame oil, and minced garlic in a small microwave proof dish and microwave for 1½ minutes on high. Remove and add soy sauce and mirin. Stir to combine.
Peel eggplant with a vegetable peeler. Cut into quarters lengthwise. Then cut into 2-inch pieces.
Place eggplants on a steaming rack and steam for 10 minutes. Remove and drain.
Pour oil and sauce mixture over eggplants. Garnish with spring onions and optional toppings (if using).