Making the Flavorful Mushroom Stuffing:
Start by heating a medium skillet over medium-high heat. Add your bacon and cook until it's wonderfully crisp and golden, about 6-8 minutes. Carefully set aside just 1 teaspoon of that delicious bacon fat in a small bowl, leaving the rest in the skillet. Transfer the cooked bacon to a paper towel-lined plate, then chop it into small, tasty bits.
In the same skillet with the remaining bacon fat, add your thinly sliced mushrooms. Sauté them until they turn a beautiful golden brown, which takes about 5 minutes.
Now, toss in the diced onion and cook it until it's tender and translucent, roughly 2-3 minutes.
Stir in the fresh baby spinach, Italian seasoning, and those chopped bacon bits. Mix everything together well and cook for just 1 minute until the spinach gently wilts. Scoop this vibrant mushroom and spinach mixture into a mixing bowl to cool slightly.
Preparing Your Pork Loin:
Pat the pork loin completely dry with a paper towel. Place it on a clean work surface. Using a long, sharp knife, carefully slice lengthwise down the center of the meat, making sure to leave about 1 inch uncut so it stays connected like a book.
Gently open the pork loin from the cut side. Place a layer of plastic wrap over the meat and use the flat side of a meat mallet to gently pound it, creating an even thickness across both halves.
Lightly brush the pork loin evenly with Dijon mustard. Then, spread your prepared mushroom stuffing evenly over the pork, leaving a small ½-inch border on all sides.
Tightly roll the pork loin, neatly enclosing the delicious filling inside. Secure it firmly with kitchen twine in three places to keep your stuffed pork loin perfectly shaped.
Cooking Your Stuffed Pork Loin to Perfection:
Preheat your oven to a cozy 375°F (190°C).
Season the rolled pork loin generously with salt and pepper, then rub it all over with that reserved 1 teaspoon of bacon fat for extra flavor and a beautiful crust.
Heat a large ovenproof skillet over medium-high heat. Carefully place the stuffed pork loin in the hot skillet and sear it until it's beautifully browned on all sides, about 6-8 minutes total.
Transfer the skillet (or the pork onto a baking sheet if your skillet isn't oven-safe) to your preheated oven. Bake for about 40 minutes, or until a meat thermometer inserted into the thickest part reads at least 145°F (63°C).
Let the pork loin rest for 10 minutes before slicing. This helps keep it wonderfully juicy and tender. Slice into beautiful rounds and serve with any pan juices!