Pat the pork belly dry with kitchen paper and score the skin with a sharp kitchen knife (or DIY knife), if your butcher has not already done so. The scoring should be right across the width of the meat, in parallel lines close together. The scoring should go through the skin, into the fat, but not into the meat below.
Preheat the oven to 220C/425F/Gas 7.
Chop together the rosemary leaves, fennel seeds, pepper, lemon zest and salt. Rub the pork all over with the sunflower oil before rubbing the herb and salt mixture into the skin, pushing it in between the scored lines. Place the pork onto a rack sat in a roasting tin, skin-side up, and cook for 35-40 minutes or until the skin starts to puff and crisp at the edges.
Meanwhile, prepare the fennel. Remove the stalks of the fennel and set aside any feathery fronds. Slice the bulb into thin wedges. Flatten the garlic and remove the peel, leaving the cloves whole.
Remove the pork belly from the oven and turn the oven down to 160C/300F/Gas 2. Lift the rack off the roasting tin and add the fennel, garlic and white wine to the pork fat in the roasting tin and stir to coat. Set the rack back on top. Return the pork and fennel to the oven and roast for a further hour at 160C/300F/Gas 2.
Remove the fennel from the oven, when it is tender and golden brown (this will take approximately 30-40 minutes). Squeeze over some lemon juice, then cover and set aside in a warm place.
To finish the pork, turn the oven back up to 200C/400F/Gas 6. Cook the pork for another 20 minutes, or until the skin is very crisp. Slice the pork belly and serve the fennel alongside, sprinkling over the chopped reserved fennel fronds for garnish.