Arrange a rack in the lower third of the oven and heat the oven to 350°F.

Trim any excess surface fat from 1 (about 3-pound) boneless beef chuck roast, then pat dry with paper towels. Season all over with 2 teaspoons kosher salt and ½ teaspoon black pepper.

Heat 1 tablespoon neutral oil in a 5-quart or larger Dutch oven over medium heat until shimmering. Add the roast and sear until well-browned on 2 sides, about 7 minutes per side. Transfer to a plate.

Add 1 diced medium yellow onion to the pot. Cook, stirring occasionally and scraping up the browned bits from the bottom of the pot, until the onions are softened, about 5 minutes. Sprinkle with 3 tablespoons all-purpose flour. Stir to coat and cook for 1 minute to cook the floury taste out.

Stir in 1 (32-ounce) carton low-sodium beef broth and scrape up any browned bits from the bottom of the pot. Stir in ½ cup balsamic vinegar, 2 tablespoons maple syrup, 3 minced garlic cloves, 1 ¼ teaspoons dried rosemary, and ¾ teaspoon dried thyme. Return the roast and any accumulated juices to the pot.

Cover and transfer the pot to the oven. Cook for 2 hours. Uncover and flip the roast. Return to the oven and cook uncovered for 30 minutes. Flip the roast once more and cook in the oven until the roast is very tender and a knife slides easily into the center of the roast, 15 to 30 minutes more.

Remove and discard the thyme stems if using fresh thyme. Shred the roast right in the pot into bite-size pieces with tongs, or transfer the roast to a cutting board and cut across the grain into thick slices (it will be easier to cut if cooled slightly first). Serve with the sauce, garnished with chopped fresh thyme and rosemary leaves if desired.