Start the day before by placing the pork on a plate, uncovered, and skin side up on the bottom shelf of the fridge. The dry environment of the fridge will dry the pork skin to help develop a great, crispy crackling.
Preheat the oven to 250°C.
Take the pork from the fridge and allow it to come to room temperature. Rub olive oil on the meat side, season with salt and pepper and the ground fennel seeds.
Place the meat on rack in a baking tray meat side down. Rub olive oil over the skin and season with salt.
Place pork in the oven and roast for 30 mins. You should see the crackling form in this time.
After 30 mins turn the oven down to 150°C and roast for a further 2 hours.
Once the oven is 150°C place the onions with skin on a baking tray and put them in the oven with the pork.
Roast the onions for 2 hours. Once cooked, remove from the oven and allow to cool.
Using a pair of kitchen scissors cut the onions open. The inside of the onions will be very soft and easily scooped out with a spoon.
Put the onion flesh into a blender and add tablespoon of butter. Blend until smooth and season to taste. This puree can be kept warm to the side.
Once the pork is cooked remove from the oven and rest for 15 mins. Do not cover with foil as steam will develop and soften the crispy skin.
Slice the pork into thick slices. Cut the pork skin side down to make this easier.
Serve on a plate with a dollop of onion puree and some steamed broccolini and potatoes.