Heat a large skillet over medium heat. Add the bacon and cook until crisp.

Remove the bacon from the pan, set aside, and drain all but 1 tablespoon of the bacon fat.

Add the mushrooms to the skillet, season with a pinch of salt and cook until the liquids cooked out of the mushrooms and they are dry 12-15 minutes.

Add the onions and let them cook until soft for the last 5 to 7 minutes.

Add the garlic and cook 30 seconds longer.

Off heat, while everything is still hot, add the rosemary and bacon to the mushroom mix and stir to combine.

Allow the mushroom filling to cool before stuffing the pork.

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.