Preheat the oven to 350 F.
In a large Dutch oven or heavy pot, heat oil over medium-high heat. Season brisket with pepper. In batches, cook meat, turning once in a while until dark brown on all sides. Put brisket on a plate.
Add onions to pot. Cook and stir until browned. Add garlic and thyme and cook and stir until fragrant, about 1 minute.
Add tomatoes, vinegar, and stock or wine. Bring to a boil. Return brisket o pot, cover, then place pot in oven. Cook until beef is tender, 3-3 1/2 hours.