Preheat the oven to 230°C.
Gently heat a roasting pan and pour in the oil from the anchovy tin, and add the olive oil.
Fry the onion and garlic for two to three minutes.
Chop the anchovies and add them, with the chestnuts, to the pan.
Add a good grind of fresh black pepper and pour in the beer.
Score the rind of the pork with a sharp knife, slicing the scores one centimetre apart, and rub with salt and oil.
Set the pork in a roasting pan so it covers the chestnut mix.
Place in the oven and roast for 30 minutes.
Then turn the temperature down to 170°C and roast for a further 30 minutes.
Serve the pork sliced with the chestnut mixture spooned over the top.
