Pierce the pork with a sharp knife, making several slits all over the top of the joint. Place the joint in a large bowl.
In a blender or food processor, puree the garlic, onion, 1/2 the cider, salt and pepper till smooth. Pour this mixture over the pork joint in the bowl. Cover with cling film and place in the fridge to marinate for 6 hours or overnight.
Remove the pork from the fridge and allow to come to room temperature. Place in a roasting tin, reserving the juice that has collected in the bowl. Combine the juices from the bowl with the reserved pineapple juice.
Insert almonds and prunes into the slits on top of the joint and in the centre. Using a baster, inject some of the remaining cider into the joint, then pour over any that's left. Rub the lard over the top of the joint, then cover with pineapple slices. Cover the roasting tin with aluminium foil.
Bake in a preheated 180 C / Gas 4 oven for 5 to 6 hours. Baste with reserved juices several times while baking. Remove the foil for the last 30 minutes in the oven to let the joint brown.