Place the spinach in a food processor. Whirl, stopping as needed to scrape down the sides of the bowl, until the spinach is finely chopped and moist but not pureed. (This step can also be done with a large, sharp knife.) Set aside.
Place the potatoes and garlic in a saucepan. Add cold water until the level is 2-inches above the potatoes. Set over medium-high heat until the water boils, then reduce the heat and cook until the potatoes are very soft, 20 to 25 minutes, depending on their size.
Drain the potatoes and garlic in a colander, then immediately return them to the hot pot, shaking the pot until the potatoes look dry. With a fork, roughly mash the potatoes to break them up. Add the spinach, scallions and oil. Mash until the potatoes are fluffy and bright green, with the skins well mixed in. The spinach will be wilted rather than soft. Season to taste with salt and pepper. Serve immediately.