In a large skillet (prefer cast iron, but any oven-safe pan will work), add 2 T olive oil over medium heat. Add chopped bacon and cook until browned. Add sliced mushrooms and onion – sauté until soft ~5 minutes.
Season with 1 t salt, ½ t pepper, parsley and garlic. Cook for 1 minute, then set aside.
Butterfly pork loin roast – if you’re uncomfortable doing this, you can always ask a meat manager to do this for you! Cover butterflied meat with plastic wrap and use a meat mallet (or saucepan) to flatten roast to ~1 inch thick all over. Be careful not to tear through the meat.
Spread the bacon mixture over the flattened loin, leaving ½ inch at the borders. Roll tightly hot-dog style and secure with kitchen twine.
Season with remaining salt (1 t) and pepper (1/2 t).
In the same skillet as the filling, add 2 T olive oil over medium heat. Once oil is hot, place loin in the skillet and sear ~2 minutes per side, or 8-10 minutes total.
Transfer in the same pan to the oven and bake at 400 degrees for 45 minutes – 1 hour, or until a thermometer reads 145 degrees F in the thickest portion of the meat.
Remove from oven and let rest for 5 minutes, then cut into ½ inch slices and serve!