Add 1 tbsp of oil to the pan. Add the onion, carrot, celery, and sweet potato to the pan and cook for 2 minutes. Add the garlic, oregano, thyme, and chili flake. Cook for 1 minute. Add the flour and cook for 1 minute. Add the tomatoes and chicken stock. Scrape the bottom and add the short rib back to the pan. Bring to a simmer. Cook until vegetable and meat are tender. The sauce should be thick. Remove from heat and season to taste.