Heat 1 tbsp of the oil in a small skillet over medium heat. Add red onion and cook, stirring regularly, until soft. Scrape into blender, add walnuts, 2 tbsp of the cooking liquid and red miso and purée until creamy, adding more of the liquid if necessary. With blender running, drizzle in the remaining 1/4 cup of oil until incorporated. Scrape into a bowl.