Place the oxtails, wine, thyme and onion into a pot and cover with water. Simmer for about 2 hours, skimming often. Remove the oxtails and, when cool enough to touch, take the meat off the bones and shred. In the meantime, reduce braising liquid to a glace and strain through a fine sieve. Heat olive oil over medium heat and saute the carrots, celery and shredded meat. When the vegetables are tender, add garlic, parsley and season with salt and pepper. Add a little of the strained braising liquid to moisten. Stuff each pepper with a little of the meat mixture. Serve right away with a drizzle of the glace and garnish with parsley.