Place 2 pounds small red potatoes and 4 smashed garlic cloves in a pot; cover with salted water. Bring to a boil, then simmer until tender, 15 to 20 minutes. Reserve 1 cup cooking water; drain. Mash the potatoes with 1¿2 cup each cooking water and buttermilk, 3 tablespoons butter, 2 tablespoons chopped dill and 2 sliced scallions, adding the remaining cooking water as needed. Season with salt and pepper.