Using a chefs knife, trim the unpeeled potatoes into rectangular blocks. Cut each into 1/4-inch thick slices. Stack 2 or more slices together and cut them into 1/4-inch julienne strips. Cut these strips into 1/4-inch dice and place in a saucepan, covered with cold water, until ready to cook. Place the potatoes in cold water, bring to boil, then immediately take off the heat and drain. Allow the potatoes to air dry. The potatoes may be held at this point for several hours without blackening.
Meanwhile prepare the persillade by mixing together the garlic and the parsley. If the potatoes aren't thoroughly dry, blot them with towels. Heat the oil in a very large skillet (ideally large enough to hold them in a single layer) over medium heat. Add the potatoes and cook, tossing occasionally, until they brown lightly, 6 to 8 minutes. Drain and discard the oil if you wish. Add the butter and continue sauteeing for another 5 minutes or longer to brown and crisp the potatoes completely. (the potatoes can be prepared to this point and held partially covered 1 hour before serving; if completely covered, they may develop off-taste and get soggy.
Just before serving, toss the potatoes with the garlic and parsley over medium heat for about 1 minute. Season with salt and pepper. Serve immediately.