Combine soy sauce, mirin, sake, water, vinegar, sugar, dashi powder, doubanjiang, grated ginger, and grated garlic in a pot and mix well. Bring to a boil over medium heat, then simmer for about 30 seconds. Remove from heat.
2
Mince the Japanese long onion and stir it into the pickling liquid.
3
Trim the stems from the shiso leaves.
4
Remove the stem from each eggplant, cut lengthwise in half, then cut again to create quarters. Drizzle sesame oil over all the pieces and coat evenly.
5
Arrange eggplant quarters on a skillet and grill over medium heat until golden brown on one side (about 2.5 minutes), then flip and cook the other side until also browned. Remove cooked pieces as they finish.
6
Layer the grilled eggplant and shiso leaves in a storage container: place 3 eggplant pieces, then 3 shiso leaves, and repeat. Pour the pickling liquid over everything. Cover tightly with plastic wrap and refrigerate for 1 hour.