Bring 6 cups of water to a boil in a large stockpot. Stir in the brine mixture and let it steep for about 5 minutes with the heat turned off.
Fill a large container with the ice and use a spoon to stir the brine into the ice to cool it completely. Add the pork loin to the cold brine and make sure it's fully submerged in the liquid. Cover the container and allow the pork to brine- under refrigeration- for 12 to 24 hours (or overnight).
Preheat your oven to 350°F. After the pork has brined, rinse off any leaves or peels that have stuck to surface, then pat the loin dry. Lay out the bacon strips, making sure to overlap the slices. Place the dried pork loin onto the center of the bacon slices, then wrap them around the loin. Flip the pork loin over and set it inside a large roasting pan so the ends of the bacon are held in place by the weight of the pork loin.
Roast the pork loin for about an hour. Use an internal thermometer to monitor the pork loin's temperature. Once the pork loin has reached an internal temp of 145°F transfer it to a carving board and allow it to rest for no less than 15 minutes. If the bacon is still a little pale towards the end of cooking, turn your oven's broiler to low and broil it for 1- minutes. Watch the meat carefully as it will brown and crisp up quickly under the broiler.
Make the Mushroom Gravy
In a saucepot, bring the drippings from the pan to a simmer over medium-high heat. Add the onions, garlic, and mushrooms to the pot and saute them until they begin to get soft- about 5-6 minutes.
Add the flour to the pot and cook for about 2 minutes, to remove the flour's starchy taste.
Gradually add the chicken stock one cup at a time, stirring the roux well after each addition.
Once all of the chicken stock has been added, add the rosemary to the pot and bring the gravy up to a simmer to allow it to thicken. Taste the gravy and season, as needed, with salt and pepper. Be sparing with the salt and the pork loin will have salt from both the brine and the bacon.Simmer on low for an additional 5 minutes.
Once the gravy has finished simmering and the pork loin has rested, use a carving knife to slice the pork loin. Ladle the mushroom gravy onto the pork, as desired.
Leftover may be stored in a food storage container in the fridge for up to three days.