Preheat the oven to 500 degrees F.
Place the pork loin bone-side down on a work surface. Insert the blade of a very sharp knife along the rib bones and slice down but not all the way through (stop about an inch from the bottom). This will release the loin from the ribs. Open the meat up a bit, turn your knife sideways and continue to slice through the meat, opening it up as it goes until you have a flat piece of pork (which will be rolled up back to the bones once stuffed with filling). Gently pound until even. Season all sides with salt and pepper.
In a food processor, add the parsley, dates, olive oil and garlic and pulse until a rough paste forms. Spread the paste on the inside of the loin, then add a layer of crumbled sausage followed by an even layer of baby spinach. Tightly roll into a pinwheel and secure tightly with butcher twine in between each bone.
Transfer the loin to a v-shaped rack in a large roasting pan and add 2 cups of water to the bottom of the pan to catch drippings. Roast for 20 minutes to create a nice crust. Then reduce the heat to 325 degrees F and roast until the pork registers 140 degrees F on a meat thermometer, about 30 minutes more. Place on a cutting board and lightly tent with foil for 20 minutes while you make the pan sauce.
To make the pan sauce, pour the pork drippings into a measuring cup. Skim off 2 tablespoons of the fat and add to the roasting pan (discard the rest of the accumulated fat from the measuring cup). Heat over medium-low heat over 2 burners. Add the flour and whisk until golden, scraping up any bits accumulated on the bottom of the pan. Add the white wine and deglaze for 2 minutes. Add the mustard and pan drippings and simmer until thick, whisking the whole time, about 5 minutes. Season with salt and pepper.
Slice the pork along the bone and plate with a nice ladleful of the mustard gravy. Garnish with chopped fresh parsley.
Ask the butcher to remove the chine bone when you purchase the pork loin! It's tough to do at home.