Generously season both sides of the chops with salt & pepper. Heat oil in a large skillet over medium-high heat. Brown the pork chops on both sides, about 3 minutes per side. Transfer chops to a plate and set them aside. Reduce the heat to medium-low and add onion and sage. Cook uncovered, stirring often, until the onion is softened and light brown in color, about 8 minutes. Stir in the prosciutto and cook 1 minute. Add squash, stock and mustard. Return pork chops to pan, cover pan. Simmer the chops until they reach 145-150°F, 10-12 minutes. Transfer chops to a serving platter; increase heat to high and boil sauce until it is slightly thickened. Season with additional salt & pepper, as needed. Spoon sauce over the pork and serve.