Butterfly the pork loin. To do so, hold the knife blade flat, so it’s parallel to your cutting board, and make a lengthwise cuts into the side of the meat, at the center. Stop short of the opposite end and open the pork loin like a book. If necessary, pound the meat to an even thickness.
Score the meat by making shallow slices in the top of the pork in a criss-cross pattern. Set aside.
Combine the rice; 1 peeled, cored, and chopped apple; 1/3 cup of chopped fennel; 3 minced cloves of garlic; 4 leaves of chopped sage; 1 sprig of chopped rosemary; the balsamic vinegar; and salt and pepper in a medium-sized bowl. Mix to combine. Set aside.
On a piece of parchment paper, layer the 8-10 slices of prosciutto so that they slightly overlap and form an area big enough to cover the pork loin. Top with the remaining sage. Place the pork loin, scored-side up, on top of the prosciutto. Season the pork with salt and pepper.
Top the pork loin with the rice stuffing and gently press down to pack the rice onto the pork.
Fold the prosciutto at the bottom of the pork loin upward so that it’s folded over the pork. Roll the pork upward to fully encase the rice.
Tie the pork loin securely with butcher’s twine. Tuck the remaining rosemary sprig into the twine, on top of the pork loin. If you have trouble with the butcher’s twine, you can use toothpicks to secure the seam of the stuffed pork loin instead.
Roughly chop the remaining apples, fennel bulb, onion, and celery and place them in a baking dish. Add in the remaining whole garlic cloves (paper removed) and toss with the oil. Place the pork loin on top of the apple and vegetable mixture.
Roast the pork loin until cooked through to 140°F, approximately 40 minutes. If the prosciutto isn’t crisp enough, simply transfer the pork loin to a baking sheet and broil for a few minutes, until darkened on top. Cut off the butcher’s twine (or remove the toothpicks) and serve the pork loin warm with the apples and vegetables. Use drippings to make gravy, if desired.