Soak the casing in cold running water for 5–10 minutes and then attach it to the tap and run water through to rinse it. Cook the garlic in the butter for 1–2 minutes until it is soft but do not brown it.
Work the lean pork and pork fat through the fine blade of a mincer. Put them in a bowl with the garlic, salt, pepper, nutmeg and brandy or Madeira and beat well to mix in the seasonings. Rub the hazelnuts in a rough towel while they are still very hot, to remove the skins. Chop them and stir them into the mixture. Fry a small piece of mixture and taste it: it should be highly seasoned, so add more seasoning if necessary.
Attach one end of the casing to a sausage stuffer or wide-based funnel and knot the other end. Push the meat mixture through and into the casing, easing it down towards the tied end. Shape it into a compact sausage about 40 cm (16 inches) long; do not pack it too tightly or it will burst during cooking. Twist the centre to divide the sausage in two and knot the other end. Prick any air bubbles with a pin. Refrigerate the sausage overnight, or for up to 24 hours.
To poach the sausage, first prick it several times. Put it in cold water to cover and heat to just below boiling point; if the water boils, the sausage will burst. Poach it for ¾-1 hour until a skewer becomes hot after being pushed into the centre. For serving, discard the casing and cut the sausage in fairly thin slices.
To cook the sausage without a casing, butter two