To make the toasted breadcrumbs, put 2 ounces sourdough bread in a food processor and pulse. Spread the breadcrumbs out on a pan and bake at 300 degrees until browned or toast in a skillet over medium heat. This is a beautiful centerpiece dish!
Combine port and dried cherries in a small saucepan over medium heat; bring to a boil. Reduce heat, simmer 4 minutes. Remove from heat; let stand 10 minutes. Drain cherries in a sieve over a bowl, reserving cherries and port.
Heat a medium skillet over medium heat. Add 1 tablespoon oil to the pan; swirl to coat. Add fennel, onion, and celery; cook 10 minutes or until vegetables are almost tender, stirring occasionally. Add the garlic; cook 1 minute, stirring frequently.
Combine the vegetable mixture, cherries, and breadcrumbs in a large bowl. Melt 1 tablespoon butter. Drizzle the melted butter and 1/4 cup stock over the bread mixture and toss. Stir in 2 teaspoons chopped sage, chopped thyme, 1/4 teaspoon salt, and 1/4 teaspoon pepper.
Cut horizontally through the center of the pork, cutting to,but not through, other side, using a sharp knife; open flat, as you would a book. Place the pork between 2 sheets of plastic wrap; pound to an even 1/2 -inch thickness using a meat mallet or small, heavy skillet. Brush 1 1/2 teaspoons oil over the inside of the pork; sprinkle with 1/4 teaspoon salt and 1/4 teaspoon pepper. Spread the bread mixture over the pork, leaving a 1/2 inch border around the outside edges. Roll up the pork, jelly-roll fashion, starting with the short side. Secure at 2-inch intervals with twine. ABrush the outside of the pork with the remaining 1 1/2 teaspoon of oil; sprinkle all sides of the pork evenly with 3/4 teaspoon salt and 1/2 teaspoon pepper.
Heat a large skillet over high heat. Add the pork to the pan; cook 8 minutes, turning to brown on all sides. Place the pork on a roasting rack coated with cooking spray; place the rack in a roasting pan. Pour the remaining 2 1/4 cups stock in the bottom of the roasting pan, Roast the pork at 400 degrees for 45 minutes or until a thermometer inserted in the center of the pork registers 138 degrees. Remove the pork from the pan; let stand 15 minutes. Cut crosswise into 12 slices.
While the pork is resting, pour liquid from roasting pan into a saucepan. Add the reserved port, thyme sprigs, sage leaves, and shallot; bring to a boil. Cook until the liquid is reduced to 1 cup (about 10 minutes). Combine the flour and 2 tablespoons water in a small bowl, stirring with a whisk., Add the flour mixture to the port mixture, stirring with a whisk; cook 5 minutes or until the port mixture begins to thicken. Add the remaining 1 tablspoon butter, stirring until the butter melts. Strain the sauce, discarding the solids. Stir in the remaining 1 teaspoon chopped sage, 1/4 teaspoon salt, and 1/4 teaspoon pepper. Serve the sauce with the pork.