Cut the ribs into individual portions and set aside in a large bowl. Make the barbecue sauce: heat the oil in a saucepan over low heat. Add the garlic and ginger and stir until aromatic, about 30 seconds Add the mirin and reduce over high heat for about 2 minutes. Add the remaining sauce ingredients and simmer for about 20 minutes, stirring. Arrange the spareribs in a large dish. Brush with generous amounts of the sauce and refrigerate for 12 hours or overnight. Preheat the oven to 400 degrees. Arrange the spareribs in a shallow baking pan and bake, brushing frequently with the remaining sauce, until the ribs are well browned, crisp, and cooked through, about 1 1/4 hours. Serve at once.