Heat the oven to 375 degrees F. Set the pork on a cutting board and imagine you're going to cut an upside-down letter T into it: in other words, slice half-way through, lengthwise, then half-way through to the left and the right. Open out the meat. Lay plastic wrap over it, and pound flat with a mallet. Season with salt and pepper, and set aside.
Rinse the spinach and spin dry, allowing a little water to remain clinging to the leaves. Put it in a saute pan, cover, and wilt, about 5 minutes. Lay the spinach on a clean tea or kitchen towel and squeeze dry. Chop, and set aside.
Wipe out the pan and put if back on the stove. Fry the bacon until cooked, remove to drain. Pour off all but a tablespoon or 2 of the fat and fry the onion until golden. Add the garlic and cook 1 minute. Stir through the bread crumbs and spinach. Season with salt and pepper.
Pat the stuffing over the meat, leaving a 1-inch border. Roll and tie the pork at 2-inch intervals. Wipe out the saute pan and heat the oil in it. Brown the meat on all sides, pour in the wine, and then transfer to the oven and roast until done, about 45 minutes. Remove from the oven and let the meat rest 10 minutes. Wrap for later, or slice and serve with the pan juices poured over.