Into a large French/Dutch oven heat oil over medium-high heat. Season beef with salt/pepper. Sear on all sides. Add onion and cook until softened. Add vinegar and reduce by half. Combine broth and Dijon and pour over beef. Add rosemary. Cover. Transfer to oven and cook for 3 hours OR until meat is tender.
30 minutes prior to finishing add carrots and mushrooms to French/Dutch oven. Season with salt/pepper. Remove beef, carrots and mushrooms. Transfer to serving dish.
Place French/Dutch oven over medium heat. Add cornstarch and whisk to combine. Cook until mixture thickens and reduces to desired gravy consistency. Check for seasoning and add if necessary.