1. Choose a good, floury potato such as Desirée, Maris Piper or King Edward. Slice the potato in to even sized pieces and drop in cold salted water in a large AGA saucepan and bring to the boil.
2. Parboil for 8-10 minutes, until the potatoes are starting to soften, then drain well for 5 minutes, until the steam subsides. Shake well in the colander or dry pan, to roughen the edges of each potato.
3. Heat your chosen fat or oil in an AGA roasting tin on the floor of the roasting oven until good and hot, then add the potatoes and turn so that ,they are well coated. If using several roasting tins, rotate them so that they all enjoy a spell crisping up on the floor of the oven.
4. Goose fat makes great roast potatoes, failing that a mixture of two fats gives the best results, e.g. solid vegetable fat and sunflower oil, used 50:50.

To cook ahead, pre-roast the potatoes and parsnips the day before, until they just start to colour. Drain off the fat, cool and then keep covered in the refrigerator till the following day. Bring back to room temperature for several hours before finishing off on the floor of the roasting oven for 20-30 minutes. Don’t place cooked roast potatoes in the simmering or warming ovens or they will not remain crisp. Instead, place the filled ovenproof serving dish on the AGA Toaster on the floor of the roasting or baking oven, with a Cold Plain Shelf above to prevent them over browning.