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.