Preheat the oven to 325 degrees F. Heat the oil in large skillet set over medium-high. Season the pork chops with salt and pepper; coat in the flour, shaking off excess. Brown the chops and then place in a casserole dish just large enough to hold them in a single layer. Add the onion to the skillet and cook 2 to 3 minutes. Add the wine, stock and garlic and bring to a simmer. Pour the mixture over the pork chops; sprinkle with sage. Cover and bake them for 90 minutes, or until very tender. Serve with the braising liquid spooned over top.