Preheat the oven to 400 degrees Fahrenheit. Spray a glass baking dish lightly with oil.Toss the sweet potatoes with about 1/4 cup teriyaki sauce, or enough to coat them fairly generously. If using, also add the sriracha. Transfer them to the baking dish.Bake until the sweet potatoes are tender but not overdone, about 40 minutes. Stir them a couple of times during baking.Top with green onions and serve hot.