Preheat the oven to 450 degrees. Brush the scallions with 2 tablespoons of the oil and sprinkle with salt and pepper. Place on a sheet pan and roast 10 minutes. Remove from the oven and chop coarsely. Toss the potatoes with remaining olive oil and sprinkle with salt and pepper. Roast until just a bit overcooked but not brown or dry, about 25 minutes. As soon as they come out of the oven, place the potatoes in a large mixing bowl and add the garlic, scallions, and butter. Smash with vigorously with a potato masher to achieve a rustic texture, the mixture should not be smooth. Season to taste with salt and pepper. To keep the smashed potatoes warm, put them in a bowl or pan, cover, and place the bowl in a large pan of warm water.