1. Position an oven rack in the center of the oven and preheat to 375°F. Pat the pork loin dry and sprinkle generously with salt and pepper. Add olive oil to a large oven-proof skillet and heat over medium-high heat. When very hot, add the pork and brown on all sides, about 8 minutes. Transfer to a platter.
2. Adjust the heat to medium and add 2 tablespoons of the butter. Add the pear wedges in a single layer and cook, turning once, until the pears are tender and begin to brown slightly, about 5 minutes. Transfer to the platter with the pork.
3. Stir the onions and rosemary into the skillet and cook until the onions begin to soften, about 5 minutes. Add the wine and cook until reduced by half, scraping up any brown bits on the bottom of the pan. Add the chicken broth and bring to a simmer. Place the pork back in the skillet and spoon the pears around the pork. Roast until an instant-read thermometer inserted into the thickest part registers 140°F, about 50 minutes. Transfer the pork to a cutting board; let rest at least 20 minutes while you make the pan sauce. Transfer the pears to a large platter.
4. Place the skillet over high heat and reduce the broth by half, about 6 minutes. Add the cranberries and mustard to the skillet; continue to cook until the mixture begins to thicken, about 5 more minutes. Whisk in the remaining 2 tablespoons butter and season with salt and pepper to taste. Slice the pork and serve with the sauce.
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