Preheat the oven to 425 degrees F (220 degrees C). Line a baking sheet with foil and spray with cooking spray.
Brush cut sides of sweet potatoes with olive oil. Lay cut side-down onto the baking sheet. Brush skins with olive oil.
Roast in the preheated oven until cooked through, about 35 minutes.
Meanwhile, prepare miso-scallion butter. Warm coconut oil in a small saucepan over medium heat. Add onion and ginger and cook for 3 to 4 minutes. Remove from heat and add miso, pressing miso against the saucepan with the back of a spoon to soften. Add soy sauce, brown sugar, and scallion.
Heat over low, stirring constantly, until sugar is completely melted and mixture is warmed through, about 3 minutes. Do not boil.
Remove sweet potatoes from the oven and flip over. Cut a slit into the sweet potato flesh and spoon miso butter over the top.