For the Yorkshire pudding, whisk the milk, flour, salt and egg together until bubbles form. Season with a little nutmeg. Make the fat liquid and line a mold with it. (A muffin tin with nine or possibly twelve cavities is best, then add a tiny bit of fat to each cavity).
Pour the batter into the mold or into the cavities.
Bake the Yorkshire pudding at 220 °C on the top rack for 15-35 minutes.