Reheat the oven to 350 degrees Fahrenheit.

Peel and core 1 pear and cut it into 1/2-inch cubes. In a skillet large enough to accommodate the pork loin (cast iron pan or Dutch oven works great), melt 2 tablespoons of butter over medium heat. Add half the onion and cook until the onions are translucent and softened, about 3 minutes. Add the pear and stir to combine. Add in the white wine and season with salt and pepper. Continue cooking until wine is evaporated, about 5 minutes. Remove to a bowl and allow to cool.  Wipe out skillet. (You will use for the pork.) 

Butterfly the pork loin: Lay the pork loin on a cutting board with the fat side down.  Using a sharp knife, cut lengthwise down the center of the loin, being careful not to cut all the way through. Leave about ½ inch uncut along the bottom.  Now butterfly the sides: cut lengthwise down the left side and the right side of the loin and open up the sides like a book. The meat should lie flat. If some areas are thicker, place a layer of plastic wrap on top of the pork and pound with a meat mallet until relatively even thickness.  (If in doubt, just ask your butcher to butterfly it for you!)

Season the surface of the pork with salt and pepper. Place the mixture of pears and onions and half of chopped gorgonzola on top of the pork. Close the pork by folding the sides back into center. Secure with kitchen string by tying the roast at 2-inch intervals. 

Wash the 2 remaining pears and cut each into 8 wedges. In the same skillet, melt the remaining tablespoon of butter. Add in the pear wedges and cook until browned on both sides. (You may need to do this in batches.) Remove the pears and set aside. 

In the same skillet, add the olive oil and heat over medium-high heat. Add the pork roast and cook, turning as needed, until all the sides of the roast are nicely browned, about 10 minutes.  Remove from the heat. Place roast fat side up, if not already. Sprinkle the remaining onion and rosemary sprigs around the roast. Pour the stock around the roast.  Place in the oven.  During cooking, keep an eye on the roast. If the liquid starts to dry out, add additional stock or water. Roast until internal temperature reaches 145 degrees Fahrenheit, about 30-45 minutes, depending on the weight of the pork roast. 

Remove from oven but maintain oven temperature.  Remove the roast to a cutting board. Tent with foil and allow to rest. Add almost all of the remaining gorgonzola (reserve a bit to sprinkle over the dish for serving) and the pears to the pan. Return to the oven to melt the cheese and warm the pears, about 5 to 10 minutes.  Remove from oven. 

Slice the pork roast into ½ inch slices and serve, alongside the pear wedges. Drizzle the cooking juices on top and sprinkle with the remaining gorgonzola. Serve.