Once the water is boiling, lower the heat to a simmer and cook for about 12 minutes, until fork tender. Drain potatoes, then place back in the pot. Do a preliminary mash with a potato masher, just to get them broken up. Add the blender mixture, salt and pepper and mash until fluffy. You may want to add a bit more milk, if needed. Taste for salt and pepper and serve garnished with extra scallions.