Preheat oven to 300 degrees. Salt and pepper the brisket generously on all sides. Add oil to a large skillet over medium-high heat and sear brisket, top and bottom. Place beef in a deep roasting pan with a tight-fitting lid (or foil). Add onions and carrot to skillet and sauté over medium-high heat 2 min. Add garlic and cook 1 min more. Add wine; bring to a simmer. Add broth, bay leaves and thyme and bring to a boil. Pour vinegar over brisket. Add hot broth mixture to brisket and cover with parchment and lid (or foil). Roast 4 hr until tender in center.
Remove brisket, reserving pan juices. Slice brisket 1/4″ thick. Discard bay leaves and thyme sprigs from pan juices, as well as half of the surface fat. Transfer brisket to serving platter.
Strain vegetables from pan juices (reserving all liquid) and puree all vegetables in a blender, adding pan juices, as needed, to reach desired gravy consistency. Taste and adjust seasoning of gravy, adding a little beef broth, salt and pepper if necessary. Serve gravy over sliced brisket with Roasted Spring Onions and a side of horseradish, as desired.