Place the potatoes, garlic, and 1 teaspoon salt in a large pot and fill with enough water to cover the potatoes by 1 inch. Bring to a boil and cook until fork tender, about 15 minutes. Drain, reserving 1 cup of the cooking liquid.

Heat 1/2 teaspoon olive oil in a medium skillet. Add the scallions and kale, and cook until wilted, 1 to 2 minutes. Set aside.

Use a potato masher to coarsely mash the potatoes and garlic. Use a rubber spatula to fold in the ⅓ cup olive oil, ½ cup of the reserved cooking liquid, the kale, scallions, rosemary, ½ teaspoon salt and freshly ground black pepper. Continue folding until creamy, adding up to ½ cup more cooking liquid and additional drizzles of olive oil, if desired. Season to taste with up to ½ teaspoon more salt, and pinches of red pepper flakes, if desired. Serve hot.