1. Preheat oven to 450F
2. Melt margarine. If using butter, clarify: melt it, skim off scum, and spoon the liquid butter off the milky residue.
3. In a heavy cast iron pan, pour 1/4 inch of margarine/butter and set over moderate heat. When hot arrange potatoes in layers starting from the centre. Baste with margarine/butter as you continue to layer the potatoes. There should be a 1/2 inch dome in the middle.
4. Butter/margarine a heavy sauce pan bottom and press hard on the potatoes. 
5. Place a drip pan on a rack below the potatoes. Cover and bake for 20 minutes. Uncover, and continue baking for another 25 minutes or 5 minutes or more if necessary. Potatoes are done if brown and crusty. 
6. Drain excess butter/margarine from the pan. Unmold or serve from the pan. Can be covered and kept warm for up to an hour in a 120F warming oven.