Make the short ribs: Preheat oven to 350°. Season ribs to taste with salt and pepper. Heat oil in a large, heavy ovenproof pan over high heat, then add ribs and brown on all sides. Transfer ribs to a plate. Add onions, celery, and carrots to same pan and sauté, stirring often, until lightly browned, about 5 minutes. Pour off excess fat, then deglaze pan with port, scraping any browned bits from bottom of pan. Reduce port by three-quarters until thick and slightly syrupy, about 15 minutes. Return ribs to pan, then add 6 cups veal stock and enough water to nearly cover ribs. Bring to a boil, cover with foil, and braise in the oven until meat is fork-tender, 2–2 1⁄2 hours. Allow ribs to cool in liquid, then cover with plastic and refrigerate overnight.
The next day, skim fat from the top and place pan with ribs and cooking liquid over medium heat. Cook until liquid has reduced by three-quarters, about 1 hour. Continue to cook, spooning pan juices over ribs, until juices are thick and ribs are glazed (take care not to burn the glaze; move ribs around in pan to keep them from burning). Cover with foil and keep warm.
Make the sauce: Melt butter in a medium saucepan over medium-low heat. Cook shallots for 2 minutes. Add truffles (if using) and cook for 2 minutes more. Season to taste with salt and pepper. Add 2 cups madeira, increase heat to medium, and reduce to a syrup, 15–20 minutes. Add chicken stock and continue to cook until reduced to a syrup, about 30 minutes. Add remaining 2 cups veal stock and reduce by half, about 15 minutes. Reduce heat to low; add remaining 1 tbsp. madeira. Cover and keep warm.
Make the potato purée: Warm cream in a saucepan over medium heat. Purée potatoes in a food mill into a heavy pot. Add butter. Gradually beat in cream. Season to taste with salt and pepper and keep warm.
To serve, place a spoonful of the potatoes in center of each of six plates, and place two short ribs on each mound of potatoes. Spoon sauce over meat and garnish each plate with steamed green beans, carrots, and chervil, if you like.