Preheat the oven to 220°C conventional oven/ 200°C fan-forced (425°F/gas 7).
To make the Porchetta stuffing, melt the butter and olive oil in a non-stick frying pan over medium heat. Add red onion and cook, stirring, until the onion is soft (approximately 4 minutes). Set aside to cool. Meanwhile, place the breadcrumbs in a large bowl and pour the Vinsanto over to soak for a few minutes. Add the rosemary and sage leaves. Season with salt and pepper to taste. Transfer the cooled onion with cooking liquid to the bowl with stuffing and mix until well combined.
Place the fennel seeds (1 tablespoon) into a mortar and pestle and crush. Stir in the chopped rosemary (1 tablespoon).
Place the pork on a chopping board. Roll up the Porchetta and use a toothpick to mark where the pork skin is folded on the inside. Use a sharp knife to carefully remove the marked pork skin. This is so that there is no pork skin inside of the Porchetta roast.
Use paper towel to pat the pork skin. Season the pork skin generously with salt and pepper. Use your hands to rub the salt and pepper into the skin. Place the pork skin side down. Rub the olive oil, crushed fennel and rosemary into the sides and bottom of the pork (being careful not to rub any olive oil over the skin). Spread the Porchetta stuffing evenly over the Porchetta. Roll the pork up tightly and tie with kitchen string.
Place the pork, joint side down into a baking tray and roast for 20 minutes.
Reduce the heat to 160°C conventional oven/ 140°C fan-forced (325° F/Gas 3). Cook for a further 2 ½ hours.
Remove the pork from the oven. The skin should be golden and crunchy while the meat of the pork lovely and moist. Transfer the pork to a chopping board and allow to rest, uncovered for 30 minutes before carving – buon appetito.