Heat your oven to its highest temperature.
Unwrap the pork and pat dry with kitchen paper. Carefully pierce the skin with a sharp knife to make several holes, cutting 1 cm deep.
In a pestle and mortar, crush together the fennel seeds and 1 tsp coarse sea salt. Zest your lemon and add the zest to the mortar with 1 tbsp olive oil. Grind to a coarse paste. Generously rub the fennel paste all over the pork.
Peel and thickly slice 2 garlic cloves form the bulb. Push a slice into each of the holes in the pork. Strip the leaves from the rosemary and push a few of them into the same holes.
Pop the joint into a roasting tin. Roast for 40 mins-1 hr till the skin is crisp, bubbled and well browned. 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 the leftover rosemary.
Loosely cover the roasting tin with foil. Slow cook the pork for 3 hrs, removing the foil for the final 30 mins. 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.