Preheat a covered barbecue on high (alternatively, preheat oven to 230°C).
Heat the butter in a small frying pan over medium heat. Add the leek and garlic and cook, stirring, for 5 mins or until leek softens. Transfer to a bowl. Set aside to cool for 10 mins. Add the pineapple, breadcrumbs, thyme and egg. Stir to combine. Season.
Place the pork, rind-side up, on a clean work surface. Pat dry with paper towel. Turn the pork over and use a sharp knife to make a 5cm-deep cut under the centre of the meat. Place the pineapple and leek stuffing across the centre of the pork. Roll the pork to enclose the filling. Use kitchen string to tie the pork at 3cm intervals to secure. Place, rind-side up, in a baking tray. Drizzle with the oil. Sprinkle with salt. Rub the salt over the rind and into the cuts. Roast in the covered barbecue using indirect heat for 30 mins or until rind begins to crackle (alternatively, roast in the oven for 30 mins or until rind begins to crackle).
Preheat barbecue to medium. (Alternatively, reduce oven to 180°C). Roast for a further 30 mins. Arrange the potatoes and herb butter from the packet around the pork. Roast for 1 hour or until pork is just cooked through. Transfer pork to a serving platter. Cover with foil and set aside for 15 mins to rest. Thickly slice.
Add the asparagus and spring onions to the potato mixture. Roast for 10 mins until asparagus is tender. Arrange the vegetables around the pork in the platter to serve.