Set up small ice bath.
In a pot with a lid, cook spinach over medium heat, allowing it to steam from the liquid still present on the leaves, about one minute. Meanwhile, take warm boiled potatoes, peel and run them through a food mill on smallest setting. Remove cooked spinach and shock in ice bath. Drain well and wrap in clean side towel and twist to totally dry leaves. Chop finely (and I mean FINELY) with a knife and add to potatoes. Make well in center, add flour,egg and salt and knead to form dough.
Bring 6 quarts water to boil and retain initial ice bath. Roll dough into one-inch thick ropes and cut into 1-inch long pieces. Flick off fork tines to establish classic shape and drop 10 to 15 at a time into boiling water. When they float, all together, remove to ice bath. Repeat until all are done. Remove from ice bath and drain. Dress with oil and hold covered in bowl in refrigerator until ready to use.
Recipe courtesy of Mario Batali