Step 1Heat the oven to 300 degrees. Heat 4 tablespoons of the oil in a Dutch oven or heavy roasting pan over high heat. Lightly dredge the short ribs in the flour (seasoned with salt and pepper), shake off excess and place in thepan. Brown well on all sides. Transfer to platter.
Step 2Add the celery, carrot, onion and bay leaves. Tie the thyme and rosemary sprigs together with string and add. Cook lightly until the vegetables are softened but not colored, about 5 minutes.
Step 3Add the Port and red wine to deglaze the pan. Raise the heat and cook, stirring, until the liquid is reduced by two-thirds, about 10 minutes. Add the chicken and veal stocks and bring to a boil.
Step 4Return the meat to the pan. Cover the pan and place it in the oven. Braise until the meat is fork-tender, 2 to 3 hours.
Step 5When the meat is cooked, remove the pan from oven and let the ribs cool in it, to room temperature.
Step 6Raise the heat to 375 degrees. Pierce the potatoes on all sides and bake until three-quarters cooked, 55 minutes. (Leave the oven on.) Cool, then peel and shred with a box grater into a bowl. Add the horseradish and salt and pepper to taste and mix well. Form the potato mixture into 3-inch cakes about one-half inch thick.
Step 7Heat 2 tablespoons of the oil in a skillet large enough to hold the potato cakes. Fry the cakes until crisp and golden brown on both sides. Keep warm.
Step 8Remove the meat from the liquid and cut into 2-inch pieces. Strain liquid into saucepan, discarding solids. Cook until reduced to sauce consistency, then whisk in the Sherry vinegar.
Step 9To serve, place 1 potato cake on each plate. Top with ribs and sauce and garnish with chives.