1. Preheat the oven to 325°. Spread the walnuts in a pie plate and toast for about 10 minutes, or until golden. Let cool, then coarsely grind the nuts. Leave the oven on.
2. Melt the butter in a medium skillet. Add the onion and cook over moderately high heat until softened and lightly browned, about 5 minutes. Add the ground walnuts, bread crumbs, parsley and sage and let cool. Stir in the Parmesan and season with salt and pepper.
3. Spread the pork roast open and season with salt and pepper. Spread half of the walnut mixture on the pork, then roll up the roast and tie it at 1-inch intervals with cotton string.
4. In a sturdy roasting pan, heat the olive oil until shimmering. Season the roast with salt and pepper and cook over moderately high heat until browned all over, 10 to 12 minutes. Add 1/2 cup of water to the pan. Roast the pork for about 2 hours, basting occasionally and adding 1/4 cup of water to the pan each time you baste. The meat is done when an instant-read thermometer inserted in the center of the roast registers 160°. Transfer the pork to a baking sheet and let stand for 10 minutes. Pour any pan juices into a measuring cup and skim off the fat.
5. Preheat the broiler. Discard the strings from the pork roast. Press the remaining ground walnut mixture onto the pork roast and broil 10 inches from the heat for about 5 minutes, or until the nut crust is golden and crisp. Let the pork stand while you make the sauce.
6. Set the roasting pan over high heat. Add the red wine and boil until reduced by half, scraping up any browned bits from the bottom of the pan. Add the chicken stock and any reserved pan juices and boil until reduced by one-third. Whisk the flour into 2 tablespoons of water, then whisk the slurry into the sauce in the roasting pan and bring to a boil. Cook until the sauce is slightly thickened and no floury taste remains. Strain the sauce into a gravy boat and season with salt and pepper.
7. Carve the pork into thick slices and serve with the sauce.
Make Ahead: The pork can be prepared through Step 3 and refrigerated overnight.
Wine Recommendation: The 1997 Robert Craig Cabernet Sauvignon and the 1992 Howell Mountain Vineyards Cabernet Sauvignon have enough fruit to pair with the juicy pork.