Preheat the slow cooker if necessary; see the manufacturer's instructions. Lay the bacon rashers on a chopping board and stretch each one, using the flat of a large cook's knife, until half as long again. Line the base and sides of a 15 cm (6 inch) diameter, deep heatproof soufflé dish with bacon.
Heat the oil in a frying pan, add the onion and fry, stirring, for 5 minutes. Finely chop or mince the pork and 1 of the duck breasts. Cut the second duck breast into long, thin slices and set aside. Stir the garlic and minced or chopped meat into the pan and cook for 3 minutes. Add the brandy, flame with a match and stand well back until the flames subside.
Stir in the remaining ingredients. Mix well, then press half the mixture into the bacon-lined dish. Top with the sliced duck, then add the remaining mixture. Fold the bacon ends over the top, adding any leftover rashers to cover the gaps. Cover with foil.
Place the dish on an upturned saucer in the base of the slow cooker pot. Pour boiling water around the dish to come halfway up the sides. Cover with the lid and cook on high for 5–6 hours or until the meat juices run clear when the centre of the terrine is pierced with a knife.
Lift the dish carefully out of the pot using a tea towel, stand on a plate, remove the foil top and replace with greaseproof paper. Weigh down the top of the terrine with measuring weights set on a small plate. Transfer to the refrigerator when cool enough and chill overnight.
Loosen the edge of the terrine with a knife, turn out, cut into thick slices and serve.