Heat the oven to 200°C/400°F/gas mark 6. Heat the oil in a roasting tin on the hob large enough for the shallots and sauté them over a moderate heat until golden. Season with the ground cardamom, salt and sprinkle over the sugar. Lightly caramelise the shallots for 2-3 minutes, then pour over the vinegar, marsala and 100 ml water. Bring to the boil, then transfer, uncovered, to the oven.