Instructions
Step 1
Cut the spring sweet onions vertically in half, then thinly slice them across the grain.
Step 2
Place [A] in a pot over heat. Bring to a boil, then simmer over medium heat for 2–3 minutes.
Step 3
In a separate pot, melt the butter over low heat. Add the sliced onions and slowly sauté, taking care not to burn. Season with two pinches of salt, cover, and steam over low heat for 10 minutes, stirring occasionally. Pour in half of the hot dashi from Step 2, cover again, and steam for another 10 minutes.
Step 4
Transfer to a blender and purée until smooth. Return to the pot, add the remaining dashi from Step 2, and warm on low heat. Season with salt to taste, then finish with a sprinkle of ground sansho pepper.