Fill a large saucepan with enough cold water to cover your potatoes. Add a pinch of salt and bring to a boil over high heat.
Once boiling, reduce heat to medium and simmer for 15-20 minutes, or until a fork easily pierces the outside but the inside is still firm. Don’t overcook as they will become mushy.
Drain the potatoes and return them to your saucepan. Cover with a lid and give the pot a few vigorous shakes. The soft, loose edges that this creates is the secret to crispy potatoes!
Add your oil to a deep roasting pan or casserole dish and heat in oven on 475F until smoking hot.
Remove the pan and add your potatoes, rosemary and a sprinkle of sea salt.
Turn the oven down to 425F and roast for 45-60 minutes, until very crispy and golden brown. Turn once halfway to ensure even cooking.