Preheat the oven to 425 degrees F. Pierce the sweet potatoes all overwith a fork and microwave until tender, about 15 minutes.
Meanwhile, coat a rimmed baking sheet with cooking spray. Combine the turkey, onion, panko and egg in a bowl. Add half each of the ketchup, hoisin sauce and ginger and season with salt and pepper. Mix with your hands until just combined. Form into a 9-by-11-inch rectangle on the prepared baking sheet. Mix the remaining ketchup, hoisin sauce and ginger in a small bowl with 2 tablespoons water; brush over the meat. Bake until golden and cooked through, 12 to 15 minutes. Turn on the broiler and broil until the glaze is just caramelized, 1 to 2 minutes.
Mix the butter, half of the scallions, 1/2 teaspoon salt, and pepper to taste in a small bowl. Split the potatoes and top with the scallion butter. Slice the meatloaf and serve with the potatoes. Sprinkle with the remaining scallions.