coffee braised pot roast with caramelized onions

1 4-lb beef chuck roast, (usually comes in 3-6 lb cuts)
salt & pepper
4 tsp evoo 
2 large onions, halved and thinly sliced 
4 cloves garlic, minced
1 tsp dried thyme
1 cup strong brewed coffee
2 tbsp balsamic vinegar
2 tbsp cornstarch mixed with 2 tbsp water

1. preheat oven to 300F.
2. season beef with salt and pepper. heat 2 tsp oil in an oven proof pot with lid over medium-high heat. add beef and cook, turning from time to time, until well browned on all sides, 7 minutes. transfer to a plate.
3. add the remaining 2 tsp oil to the pot. add onions, reduce heat to medium and cook, stirring often, until softened and golden, 7 minutes. add garlic and thyme; cook, stirring, for 1 minute. stir in coffee and vinegar; bring to a simmer. return the beef to the pot and spoon some onions over it. cover and transfer to the oven.
4. braise the beef in the oven until fork-tender but not falling apart, 2 1/2 to 3 hours. 
5. transfer beef to a plate. meanwhile, skim fat from the braising liquid; bring to a boil. add the cornstarch mixture and cook, whisking, until the gravy thickens slightly, about 1 minute. season with pepper. serve beef with gravy.