For the pork, preheat the oven to 230C/450F/Gas 8. Using a small, sharp knife, score the whole skin of the shoulder with deep cuts about 5mm/¼in wide.
Smash the garlic with the fennel seeds, then mix with salt, pepper and chilli to taste. Rub and push this mixture into and over the skin and all the surfaces of the meat.
Place the shoulder on a rack in a roasting tin and roast for 30 minutes or until the skin begins to crackle up, blister and brown. Turn the shoulder and pour over half the lemon juice and 2 tbsp olive oil.
Turn the oven down to 110C/225F/Gas ½ and leave the meat to roast overnight, or all day (from 8 - 24 hours). Turn over occasionally and baste with extra lemon juice and if necessary a little oil.
The shoulder is ready when it is completely soft under the crisp skin. You can tell by pushing with your finger - the meat will give way and might even fall off the bone. Serve each person with some of the crisp skin and meat cut from different parts of the shoulder. Add extra lemon juice to deglaze the pan and spoon this over.
To make the pesto, mix all the ingredients in a blender and serve.
Serve with batons of roasted carrots and parsnips which have been tossed in a little olive oil and honey and roasted for about 30 minutes at 200C/400F/Gas 6