Trim the top of garlic and pull away the outermost skin. With your hands, lightly rub the garlic with oil and place in a small roasting dish. Cover with foil.
Roast the garlic until the cloves are fork tender (but not burned), about 50 minutes. Check for doneness at 40 minutes. (A kitchen timer is helpful.)
Meanwhile, prepare the potatoes: Quarter and place in a medium-size saucepan with 4 cups of water. The water should just barely cover the potatoes. This is important.
Add the salt. Cover the saucepan and bring to a boil. Lower the heat to medium. Cook for 25 minutes, place the kale on top of the potatoes, replace the lid, and allow the kale to steam for 5 minutes. Test the potatoes with a fork for doneness. Remove from the heat.
Squeeze the garlic flesh from the cloves into a large mixing bowl. With tongs or a strainer, transfer the potatoes and kale to the mixing bowl, reserving the cooking liquid.
With a hand masher, mash the potatoes and kale, focusing on the potatoes at first, ensuring that they’re smooth, ladling in their cooking liquid as necessary (you’ll use some, but likely not all). Use a wooden spoon to combine all of the ingredients. Add the oil and black pepper, stir, then taste for salt, adding more as needed.