Cook down the cream in a small, heavy saucepan over medium heat until reduced by about a third. Set aside.
Heat a heavy cast-iron griddle or skillet over high heat until a drop of water sizzles on contact. Roast the unpeeled garlic cloves and onion, turning several times, until the garlic is dark on all sides and somewhat softened and the onion is partly blackened and fragrant. Set asid