Preheat oven to 425 degrees.
In a large bowl, combine 1/4 cup oil, soy sauce, mirin, ginger, rice vinegar, light brown sugar and garlic; add sweet potatoes and toss to coat well.
Spray a 13x9 baking dish with cooking spray and arrange sweet potato mixture in a single layer.
Cover dish with foil and bake for 25 minutes.
Meanwhile, heat remaining oil or butter in a skillet until shimmering.
Add panko bread crumbs, stirring constantly until panko is golden brown (watch the pan carefully so the panko doesn't burn).
Remove panko to a paper towel-lined plate and let cool.
Remove the potatoes from the oven and remove the foil, stirring the potatoes.
Spread back into an even layer and roast, uncovered, an additional 10 minutes.
Remove potatoes again, toss, spread back into an even layer and roast for a final 10 minutes.
Remove potatoes from the oven and let cool slightly. Toss with panko and scallions and serve immediately.