PREHEAT the oven to 450°F/230°C. TOSS the butternut squash cubes with 2 tablespoons of the oil. PLACE in a baking tray and cook for 15 minutes. TOSS the squash and cook for a further 10 minutes. WHILE the squash cooks, heat 1 tablespoon of oil in a large saucepan over medium heat. COOK the scallion for 1 minute, then add the ginger and cook for 2 minutes, stirring regularly. MIX the miso with ¼ cup of the stock, stirring to dissolve. ADD the mixture to the pan with the remaining stock, soy sauce, and roast squash. TRANSFER the soup to a food processor or blender and process until smooth. SERVE in bowls garnished with the reserved scallion.