Put the potatoes into a saucepan of heavily salted, cold water. Place the pan over a high heat and bring the water to the boil. Reduce the heat and leave the potatoes on a gentle boil for about 20-30 minutes, until they fall off the tip of a knife. Drain in a colander.
Once the potatoes are cool enough to handle, divide them into two unequal portions of one third and two thirds. Mash the smaller portion and cut the larger portion into 5mm cubes.
Heat a large saucepan over a medium heat with a splash of oil. Add the onion and fry for 4 minutes, until soft and translucent. Remove the pan from the heat and add the cubed potatoes and the mashed potato. Mix well.
Add the cornflour and season well with salt and pepper. (You can add any other herbs and spices you fancy, let your imagination go wild here.)
Line a baking tray with baking parchment.
Divide your potato mixture into 8 equal portions (about 70g each) and mould each into a hash-brown shape - I like triangles, but you do you. Place each moulded hash brown on your lined baking tray, as you go. Transfer the baking tray to the fridge and chill the hash browns for at least 1 hour to firm up.
Pour your oil into a deep-fat fryer or a heavy-based saucepan (don't let it come more than halfway up the inside of the pan). Place the pan over a high heat and get the oil to 180°C.
In batches, fry the hash browns until golden - about 3 minutes. Drain each batch on kitchen paper while you fry the remainder. Sprinkle with flaky salt to serve.