The reason this works as a great tailgating or casual party recipe is that you can do most of it ahead, and also because guests can help assemble their own and even customize. Porchetta sandwiches are best left simple in my opinion, but condiments like a grainy or Dijon mustard and add-ons like some garlicky sautéed greens or caramelized onions would be delicious as well.
For the porchetta:
Start to slice into the pork loin about ½ inch away from the end of the longest side. It should be as if you are shearing ½ inch from the hunk of meat, but then continue to cut and unravel the loin so that you have 1 long flat piece of pork that is about ½-inch thick throughout.
Season the pork on both sides with the salt and pepper. In a food processor, combine the fennel, sage, rosemary, fennel seed, garlic, olive oil, and 1 tablespoon of white wine. Spread the mixture on 1 side of the pork loin. Roll it up like a burrito and secure with two pieces of kitchen twine. Refrigerate for at least 2 hours or up to overnight.
Preheat the oven to 375 degrees.
Roast pork for 30 minutes then pour the ¾ cup of white wine over the porchetta. Roast for another hour. You should insert a meat thermometer in the thickest part and make sure that it reads at least 145 degrees. Remove the pork from the oven and let it stand for 10 minutes before slicing.
For the sandwiches:
Drizzle olive oil on both sides of the bread. Top with arugula and porchetta slices. (And if you saved the pan juices, drizzle some of them over the meat.)