For the fennel and golden raisin stuffing: Place the golden raisins in a small dish, cover with boiling water and let stand to soften. Melt the butter with a turn of the pan of EVOO in a skillet over medium heat. Add the garlic, fennel (reserving the fronds) and onions, sprinkle with some salt and pepper and cook until the vegetables soften, 12 to 15 minutes. Deglaze with the wine, transfer to a bowl and let cool completely. Drain and chop the raisins, and then combine with the fennel, onions, ground pork, pine nuts and thyme.
For the prosciutto-wrapped pork roast: Preheat the oven to 400 degrees F. Butterfly the meat by thirds: Measure one-third from the bottom of the loin, and make a lengthwise cut along one longer side (not going all the way through to the opposite side). Continue to make long cuts to open up the meat. When you have reached almost to the opposite side of the roast with your lengthwise cuts, stop and roll the top two-thirds of the roast over. Measure the height of the rolled-over portion and make a lengthwise cut to butterfly this in half. Continue slicing and opening up the meat until the loin has been butterflied into a single slab.
Pound the meat with a meat mallet to 3/4-inch thick, and then sprinkle with salt and pepper. Spread the stuffing over the meat, leaving a 1-inch border. Roll up the pork, cover with overlapping slices of the prosciutto, and tie in intervals with twine to secure. Heat 2 turns of the pan of EVOO in a large Dutch oven over medium-high heat. Add the pork and brown evenly all over.
Toss the potatoes and onions in a roasting pan together with the rosemary, garlic, about 1/4 cup EVOO, and some salt and pepper. Set the browned roast on top and deglaze the Dutch oven with the wine, scraping up the bits from the bottom. Pour the liquid over the pork. Roast until the internal temperature reaches 145 degrees, up to 2 hours, then let rest 30 minutes under a foil tent.
Before serving, place the roasting pan on a burner, deglaze with the stock and then stir in the fennel fronds and butter. Slice the roast, spoon the fennel butter jus over top and serve alongside the potatoes and onions.
NotesCook's Note: For a make-ahead meal, store the stuffed and rolled pork up to 2 days in advance in the refrigerator, then bring to room temperature before roasting.