Put the potatoes and bay leaf into a medium saucepan, cover with cold water and bring to a simmer. Cook over medium-high heat until fork-tender, 35 to 40 minutes, adjusting the heat as needed to maintain a simmerthe water should not boil. Reserve 1 cup of the potato water; drain the potatoes well, and return them to the saucepan.
Meanwhile, heat the oil in a small skillet over medium heat. Reserve 2 tablespoons of the scallions; add the remaining scallions, garlic and 1/4 teaspoon salt to the skillet, and cook, stirring occasionally, until the scallions are tender, about 5 minutes. Turn off the heat, and whisk in 3/4 cup of the yogurt.
Add the yogurt mixture, 1/2 cup of the reserved potato water, 1 1/4 teaspoons salt and 1/4 teaspoon pepper to the hot potatoes. Mash with a potato masher until mostly smooth (a few chunks are fine). Stir in additional potato water if you want creamier potatoes. Transfer to a serving dish, dollop the remaining Greek yogurt on top and garnish with the reserved scallions.
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