Heat your oven to its highest temperature.
Unwrap your pork and pat dry with kitchen paper. Carefully pierce the skin with a sharp knife to make several holes, cutting 1 cm deep. Peel and thickly slice 4 garlic cloves. Push a slice into each of the holes in the pork. Strip the leaves from the rosemary and push them into the same holes.
In a pestle and mortar, crush together the fennel seeds and 1 tsp coarse sea salt. Zest your lemon and add to the mortar with 1 tbsp oil. Grind to a coarse paste. Generously rub the fennel salt all over the pork.
Pop the joint onto a roasting tray. Roast for 1 hr. Turn the temperature down to 160°C/Fan 140°C/Gas 3. Halve the remainder of the garlic bulb and halve the lemon. Tuck both under the pork with any leftover rosemary. Slow cook the pork for 3 hrs. Cover with a layer of foil if the skin darkens too quickly.
Allow the pork to rest for at least 30 mins. The pork should be meltingly soft and the skin crispy. If the skin isn’t crisped up enough, pop under a hot grill for 2 mins.
Remove the roasted lemon and garlic from the tin. Squeeze the roasted garlic flesh into a bowl and juice the lemon. Pour in the pork juices. Mix and serve with slices of pork.