Turn on your oven and preheat to 200C/fan 180C/gas 6.
Wash and peel the potatoes and cut into sensible size pieces. I normally do thirds or quarters depending on the size of the potato.
Pour water into a large sauce pan and add the potatoes. The water should be just covering the potatoes.
Add a good pinch of salt and place over a high heat until boiling.
When it starts to boil, lower the heat and simmer the potatoes for 2 - 3 minutes.
Pour the oil into the roasting tray and place in the oven for a few minutes to get really hot.
Drain the potatoes in a large colander. The larger the better as they need room to bounce about.
Shake the colander with the potatoes in to remove any excess water but also to scuff the skins of the potatoes. This will help create a lovely crispy skin when roasting.
Add the flour to the potatoes and shake again making sure the flour has covered all of the potatoes thinly..
VERY CAREFULLY remove the roasting tray with the hot oil in it from the oven and add the potatoes. I use a large spoon to do this to avoid splashing.
Rotate the potatoes so they have oil on every side.
Sprinkle the rosemary over the potatoes.
Make sure they are in a single layer and place them in the oven for 15 minutes.
After 15 minutes, remove from the oven and turn them over. All sides will be cooked perfectly this way.
Return to the oven and roast for a further 15 minutes.
After 15 minutes, they should be lovely and golden and have a crispy texture. If they are not quite ready put them back in for a further 10 minutes.
When ready , remove from the pan and serve immediately.
Sprinkle a little rock salt over them prior to serving. It really adds another amazing flavour.