I like to cook these aromatic potato parcels in the embers of the bonfire on Guy Fawkes night but they are also very good cooked on a barbecue.
Bring a pan of salted water to the boil, add the potatoes and shallots and reduce the heat to a simmer. Cook for 10-15 minutes, then drain well. Place in a bowl, season with sea salt and black pepper and add the olive oil and rosemary.
Wrap the potatoes and shallots loosely in 2 or 3 layers of thick foil and place amongst the coals of a barbecue or bonfire at medium temperature. Turning the pouch regularly, cook for about 20 minutes or until the potatoes and shallots are tender and the skins somewhat crisp. Open up the parcel and top with the butter, which will melt over the vegetables. Serve immediately.