Remove the chuck roast from the refrigerator and let it sit at room temperature for about 30 minutes.
Once at room temperature, pat the roast thoroughly dry with paper towels, then season all over with kosher salt and black pepper.
Preheat the oven to 300°F.
Heat olive oil in a large Dutch oven or a large pot with a tight-fitting lid over medium-high heat. Once the oil is hot, add the roast and sear for 3 to 4 minutes per side until a deep brown crust forms. Transfer the roast to a plate.
Reduce heat to medium. Add the sliced onion to the pot and cook for 4 to 5 minutes, scraping up the browned bits from the bottom.
Add the smashed garlic and cook for 30 seconds.
Pour in the beef broth and Worcestershire sauce, stirring well to combine. Add the fresh thyme sprigs, if using.
Return the roast to the pot along with any accumulated juices.
Check the liquid level; it should come about halfway up the sides of the roast. If needed, add a bit more beef stock to reach that level.
Cover with a tight-fitting lid and transfer to the oven. Cook for 3 ½ to 4 hours, flipping the roast once halfway through, until fork-tender and easily pulled apart.
Remove the roast from the oven and let it rest, uncovered, for 10 to 15 minutes. Then, remove the roast from the pan, and gently pull it apart.
Serve warm with mashed potatoes and mushroom shallot gravy.