Preheat the oven to gas 8 / 230C / 450F and ready a rectangular ovenproof dish
Mix the margarine and oil together and lightly grease the bottom and sides of the ovenproof dish with a little of it
To the rest, add the chopped garlic, rosemary, salt and pepper
Peel the potatoes and slice them as thinly as you can (you can leave the skins on if you prefer.) A mandolin is a handy piece of equipment for this but watch your fingers!
Take stacks of the potato slices and place in vertical rows in the dish. Don't overcrowd it. Use a bigger one if needed rather than packing the potato slices in or else they won't cook through evenly
Drizzle with the remaining oil mixture, making sure you get some inbetween the potato slices. Add a sprig of rosemary to the dish for extra flavour
Bake for 70 - 90 minutes. If the edges are starting to overbrown before the rest of the slices are cooked, cover the dish with foil
When done the edges should be crispy but the potato slices themselves should still be quite soft
Sprinkle with extra salt before serving
