Preheat your oven to 210c fan / 230c. 

Add the potatoes to a large pan and fill up with cold water. Add ½ tsp salt to the pan and place it on a medium-high heat.

 
Whilst the potatoes are boiling and once the oven is hot, pour the oil into a large, shallow baking tray and pop it into the oven for around 20 minutes until the oil is smoking hot. A shallow baking tray will allow the potatoes to crisp up rather than steam.

 
Bring the potatoes up to the boil the simmer for 8 minutes so that the potatoes soften but not enough to completely crumble. Drain them through a colander, then shake them around so that their edges really fluff up. Pop the potatoes back into the dry saucepan, sprinkle in the cornflour, pop the lid on the pan and shake the potatoes around again a little so the cornflour fully disperses into the fluffy edges of the potatoes.

 
Take the tray out of the oven, and carefully, one by one, add the potatoes to the smoking hot oil, evenly spacing them around – do not crowd them too close together otherwise the potatoes will end up steaming rather than roasting and crisping. Once all the potatoes are in the tray, very carefully tilt the tray and spoon the hot oil over the tops of the potatoes. Sprinkle liberally with the remaining sea salt, then place the tray back in the oven on the middle shelf.  Turn the heat down to 180c fan / 200c.

After 20-25 minutes, or when the bottoms and sides of the potatoes starting to crisp up, take the tray out of the oven, turn the potatoes and give them a gentle toss in the oils. Add the fresh herbs to the tray then pop the tray back into the oven. Keep repeating this basting and turning process every 20 minutes or so for around another hour, until delectably rich golden brown in colour, super crispy on the outside, and irresistibly fluffy on the inside.