Preheat the oven to 375.
Butterfly the pork and arrange it flat.
Layer on the mushroom filling to coat the insides and roll the pork back up, tieing as needed to secure. Season with salt.
Meanwhile, heat a skillet over medium-high heat.
Add the olive oil. And swirl to coat.
Place the pork in the hot pan, fat side down, and allow the pork to brown and the fat to crisp, 5 to 7 minutes.
Brown all sides, about 5 minutes.
Place pork in the skillet with the fat cap on top, add fresh rosemary to the top if desired and toss in a few extra garlic cloves if you have them.
Place the pork in the oven and roast for about 50 - 60 minutes, until the internal temperature reads 145 with an instant-read thermometer.
Remove from the oven, cover and allow to rest 10 minutes before slicing to serve.