Make stuffing. In a large skillet over medium heat, sauté bacon until brown and crisp; this may take about 6-7 minutes. If the bacon fat is more than 2 tablespoons, reserve the extra for later use.
Add mushrooms, salt, and pepper to taste; cook until the mushrooms are soft, about 4-5 minutes more minutes. Remove and set aside.
In the same skillet, add 2 tablespoons olive oil, onions, garlic and thyme and sauté for 4-5 minutes or until onions are tender. Return the mushroom and bacon to the onion mixture and cook for about 2 minutes. Remove from heat.
Mix in breadcrumbs and parsley. Stir to combine. Let it cool and set aside.
Rinse the pork and pat dry with paper towels.
Lay the pork loin on a large cutting board with fat side up. With a sharp knife parallel to the cutting board, make a 1-inch-deep incision down the length of each side, one side higher; avoid cutting the meat all the way through. Open the meat like a book so the pork loin lies flat. Cover the pork with plastic wrap or parchment baking paper and then pound with a rolling pin or a meat tenderizer to flatten the pork.
Season with salt, pepper and Italian seasoning. Spread the mushroom mixture over the pork loin. Then roll the pork like a jelly roll, starting with a long side; tie several times with kitchen string, or use toothpicks to seal the pork to secure the filling. Season the outer layer with salt and pepper.
Heat remaining oil in a large skillet or cast iron over high heat. The skillet should be large enough to fit the pork. Add pork and brown the pork. Use tongs to move the pork around. Give it at least 2 minutes per side, so it browns up nicely. It will take about 6-8 minutes.
Transfer pork to a roasting pan, Bake in the preheated oven for about an hour until an instant-read thermometer inserted into the thickest part reads 155℉. Let it rest for 5-10 minutes, and then transfer to a cutting board. Remove the kitchen twine and slice.
