Preheat your BBQ grill to medium-high heat. Wash the Japanese sweet potatoes thoroughly under running water to remove any dirt.
Pat them dry with a kitchen towel. Cut the sweet potatoes into approximately 1-inch thick rounds or wedges, depending on your preference. Keeping the pieces similar in size will ensure even cooking.
In a mixing bow, combine the vegetable oil, soy sauce, mirin, honey or maple syrup, sesame oil, grated ginger, minced garlic, chili flakes, and a pinch of salt. Whisk marinade until well combined.
Place the sweet potato pieces in a large resealable bag or a shallow dish. Pour the marinade over the sweet potatoes, ensuring they are evenly coated. Seal the bag or cover the dish and let the sweet potatoes marinate for at least 30 minutes, or up to 2 hours for more flavor.
Place the potatoes on the grill turning them at half way point after 6 minutes. (each side) You can use the juice to put on the potatoes while grilling.
