Preheat the oven to 325 degrees F.
Heat the oil in a Dutch oven or heavy-bottomed pot over medium-high heat. Season the roast lightly with salt and pepper. Put the roast fat-side down in the pot. Cook until the bottom is browned, about 3 minutes, then carefully flip the roast to brown on the other side, an additional 3 minutes. Remove the meat to a plate.
Drain off all but 2 tablespoons of fat from the pot. Add the onions and stir periodically until they soften and start to brown, about 8 minutes. Stir in the onion soup mix, thyme, bay leaves and 2 cups water. Add the meat with all its juices back to the pot. Bring the liquid to a simmer. Cover, transfer the pot to the oven and cook until the meat is very tender but still sliceable, 2 1/2 to 3 hours.
Transfer the meat to a cutting board. Discard the thyme sprigs and bay leaves from the braising liquid and skim off any excess fat. Slice the meat across the grain into slices 1/2 to 1 inch thick, spoon the braising liquid over and serve hot.