1. Preheat oven to 325°F (160°C). Season beef short ribs with sea salt and black pepper, then dust evenly with all-purpose flour. Heat 2–3 tbsp olive oil in a large Dutch oven over medium heat. Sear short ribs in batches until browned on all sides, then transfer to a plate.
2. Add chopped onion and a splash of olive oil to the same pot. Cover and cook gently for 4–5 minutes until softened and lightly caramelized. Add minced garlic, carrots, and celery; cook for 5 minutes longer, lowering heat if needed.
3. Return short ribs to the pot along with tomato paste. Pour in dry red wine, stirring to deglaze the bottom of the pot, and cook for 1 minute. Add 2 cups water, Beef Bone Broth Concentrate Natural, brown sugar, 2 tbsp balsamic vinegar, and fresh rosemary sprigs. Place a piece of parchment paper directly over the meat, cover tightly with the lid, and braise in the oven for 3 hours.
4. Once the beef has braised for 90 minutes, prepare the onions: peel red onions, trim roots slightly while keeping base intact, and cut into eighths through the root so wedges remain whole. Place on a rimmed baking sheet, drizzle with 2 tbsp balsamic vinegar and olive oil, season with salt and pepper, and toss gently. Cover tightly with aluminum foil and bake for 1 hour. Remove foil, carefully flip wedges, and roast uncovered for 30 minutes until tender and caramelized.
5. Transfer braised beef to a platter with a ladle of braising liquid to keep moist; tent with foil. Strain remaining liquid into a saucepan without pressing vegetables. Whisk in Dijon mustard and remaining 2 tbsp balsamic vinegar, then simmer over medium heat until thickened into a glossy syrup.
6. Arrange short ribs and roasted red onion wedges on a serving platter and spoon warm glaze over the top.