In a medium pan, melt the butter and add the chopped onion, mushrooms, and apple.
Saute them for 2-3 minutes.
Add the bread or the bread crumbs and mix.
Season with salt, pepper, and nutmeg. Add the lemon zest.
Remove the pan from the stove and set it aside.
In the meantime, soak 6-7 toothpicks in water to prevent them from burning. Rinse the pork and pat dry.
Then, make a 1-inch-deep incision down the length of the loin; do not cut all the way through. Open the meat like a book, so the meat lies flat.
Cover the pork with plastic wrap and pound it with the flat side of a meat mallet until about ½ inch thick, starting from the middle and working outward. This will tenderize the meat and make it a bit easier to roll.
Spread the apple and mushroom mixture over the surface of the pork loin.
Starting with a long side, tightly roll up the pork loin. Secure the seams with the toothpicks.
Meanwhile, preheat a large pan.
Brush the pork loin roll with olive oil and season with salt and pepper.
Place the meat in the pan and lightly brown all sides for 4-5 minutes.
Transfer the meat to a baking pan.
Deglaze the pan where you browned the meat with a cup of water or broth. Add salt and pepper to your taste and pour the sauce over the meat. Add the bay leaf to the sauce.
Roast the pork loin for about 1 hour at 350F(180C) or until the temperature inside the thickest part of the meat is 155F(68C).
Remove from the oven and let it rest for a few minutes.