Heat the olive oil in a large saucepan over medium-high heat. Season the oxtails with salt and pepper, and sear them on all sides until browned. You may need to do this in batches. Transfer the oxtails to a plate and set aside.
Add the onion, garlic and chili to the same pot and cook until translucent and slightly charred, about 3-5mins. Add in the tomato paste, and cook for 2-3 minutes until the paste browns slightly.
Stir in the beef broth, wine or vinegar, thyme, bay leaves, carrots, celery, salt, pepper and cooked oxtails. Ensure there’s enough liquid to cover the oxtails. Turn the heat to low and allow to simmer. Cover with a lid slightly cracked. Simmer for 2.5 to 3 hours.
About 1 hour before serving, remove the lid. And allow the stew to continue cooking uncovered.
30minutes before serving, mix 1 tbsp cornstarch and 1 tbsp water together and stir the slurry into the pot and increase the heat to medium, stirring until the gravy thickens. Salt to taste.
Garnish with chopped parsley and start serving!