Clean the livers very carefully, paring off any greenish parts, which may impart a bitter flavour to the pâté.
Melt 50g of the butter in a frying pan and sauté the grated onion until soft, then remove it from the pan and put it to one side.
Add a little more butter if necessary and then sauté the livers, whole, for about 5 minutes (they should be cooked on the outside but still pink in the centre).
Remove the livers from the pan and pour in the brandy.
Heat the brandy until it bubbles, then add the port, garlic, mustard, thyme and mace, and cook for a minute.
Put the livers, onion, pan sauces and remaining butter into a food processor or blender and blend to a smooth paste, then add salt to taste.
Spoon the mixture into small pots and top with clarified butter to seal: the butter should be 3–5 mm thick.
Cover the pots with foil and chill the pâté thoroughly before eating.