Pat the beef chuck roast completely dry with paper towels. Season it aggressively on all sides with the kosher salt and black pepper. Heat the olive oil in your white enameled Dutch oven over medium-high heat. Carefully lower the roast into the pot. Let it sear undisturbed for 4-5 minutes per side until a deeply browned, dark crust forms. Remove the beef and set it aside on a plate.

Lower the heat to medium and melt the unsalted butter in the same pot. Add the massive pile of sliced yellow onions. Cook them slowly, stirring occasionally, for about 25-30 minutes until they turn a deep golden brown and transform into a heap of sweet caramelized onions. Stir in the minced garlic during the last minute of cooking.

Sprinkle the all-purpose flour over the caramelized onions. Stir continuously for 2 minutes to cook off the raw flour taste. Pour in the dry sherry or red wine to deglaze the pot, scraping up all the dark brown bits from the bottom. Slowly whisk in the beef broth and bring the mixture to a gentle simmer.

Preheat your oven to 300°F (150°C). Return the seared beef to the Dutch oven, resting it gently in the simmering onion gravy. Tuck 4 sprigs of fresh thyme into the liquid. Cover the pot with its heavy lid and transfer to the preheated oven. Braise for 3 to 3.5 hours, or until the meat is incredibly tender and yields easily to a fork.

Remove the pot from the oven. Transfer the beef to a cutting board and cover loosely with foil to rest for 15 minutes. If needed, simmer the gravy on the stovetop over medium heat for 5-10 minutes until it glistens. Discard the cooked thyme stems. Serve the beef smothered in the rich dark brown gravy, topped with the sweet caramelized onions, and garnish with a fresh sprig of fresh thyme.