Begin by pounding the pork tenderloin to a ½-inch thickness. Sprinkle with salt and pepper, then layer on the pear and Gruyere.
Lay out the slices of prosciutto on a piece of plastic wrap so that they mimic the shape that the tenderloin has taken. For mine, I made two columns of three slices of prosciutto each, right up next to each other. Place the pork on top of the prosciutto.
Using the plastic wrap to hold everything together, first roll up one side of the tenderloin, then the other, having the two ends of the pork meet at the top, with the prosciutto extending over that seam just a bit.
Using a piece of butcher’s twine, make loops down the length of the tenderloin to hold everything together. I just kind of made up my method, but use whatever works best for you while losing as little of the filling as possible.
Preheat the oven to 350 degrees Fahrenheit for the onions.
Place a large, deep saute pan or Dutch oven on the stove over medium-high heat and melt the butter in it. Once the butter is melted and the pan is nice and hot, add the pork and brown it on all sides (about 2 minutes each side.)
Once the pork is browned, pour in the sherry and let the alcohol evaporate off a bit, about 3 minutes. Add the chicken broth.
Cover the pot or pan and cook for about 15 minutes.
While the meat is cooking, prep the onions. In an 8-by-8-inch baking dish, spread out the onions in a thin layer. Cut the butter into tablespoons and drop the tablespoons evenly over the onions, followed by the sugar, white vinegar, salt and pepper. Place in the preheated 350 degree oven and cook for 30-40 minutes, or until the onions are golden brown and can be pierced easily with a fork.
After 15 minutes, remove the lid and pour off the liquid until only about ½ cup remains. Flip the pork over and replace the lid. Cook the pork for another 15 minutes, or until cooked through.
Transfer the pork to a serving dish and surround it with the cooked cippolini onions. Garnish with whatever fresh herbs you may have on hand!