As seen on: Giada's Holiday Handbook, Season 2, Episode 5. Christmas at Giada's.
Preheat the oven to 425 degrees F. Peel and slice the onion into 4 rounds. Place in the bottom of a 9 by 13 pan and set aside.
Using the tip of a sharp knife, score the fat on the top of the loin in a diamond pattern, about 1/4 inch deep. Unroll the pork loin so it is completely flat. Season the inside with 1/2 teaspoon salt. Lay the provolone slices over the pork, shingling slightly. Open the peppers so they lay flat. Tear or cut them in half so you are left with 8 triangles. Lay the peppers flat on the cheese fitting them together like a puzzle. Spread the spinach evenly over the peppers. Beginning at one end, roll the pork back up so that it is in the original loin shape with the fat on top. Tie the loin in 4 places using butchers twine. Season it all over with the remaining salt. Place the pork on the bed of onions in the prepared pan. Scatter the rosemary sprigs around it and pour the broth in the bottom of the pan. Drizzle with the olive oil. Roast for about 50 minutes or until the top is golden brown.
Baste the pork with the juices and reduce the heat to 375 degrees F. Continue roasting for approximately 40 minutes longer or until an instant read thermometer inserted in the center reads 150 degrees F. Allow the pork to rest for 15 minutes before slicing. Strain the pan juice and serve along side the pork.