Preheat the oven to 220°C / 425°F / Gas Mark 7.
Peel and halve the potatoes.
Boil some salted water. When the water is at a rolling boil, drop the potatoes into the pot.
Simmer them for 10 minutes. Strain the potatoes immediately.
Give the potatoes a good shake in the pot to roughen up the outsides. This will make the outside the potatoe lovely and crunchy.
Melt half of the butter and goose fat in a roasting tin on top of the stove.
When it is hot, add the potatoes and allow them to cook on top of the stove on a low heat until the potatoes are golden brown on all sides. It's just for a few minutes.
Add the rest of butter and goose fat if you feed the potatoes are sticking.
Sprinkle the rosemary over the potatoes and roast in the oven for 15 to 20 minutes.
If you have time, turn them at least once during that roasting time for a perfect roast potato.
To serve, sprinkle with a little flat leave parsley and coarse sea salt.