Pat the beef chuck roast dry with paper towels and season generously all over with 2 teaspoons of kosher salt and the freshly ground black pepper—this helps create a better crust when searing. While the roast rests, thinly slice all 6 onions and mince the 4 garlic cloves; set them aside separately. Heat 1.5 tablespoons of olive oil in a large, heavy-bottomed Dutch oven over medium-high heat until shimmering. Sear the roast for 4-5 minutes per side until deeply browned on all surfaces, then transfer to a plate. Lower the heat to medium, add the remaining 2 tablespoons of olive oil, then add the sliced onions with the remaining 1.5 teaspoons of salt and ¼ teaspoon of black pepper. Cook covered for 5 minutes, stir well, cover again, and cook for another 5 minutes to release the onions' moisture.

Uncover the pot and continue cooking the onions over medium heat, stirring frequently every 3-4 minutes, until they're deeply caramelized and golden brown, about 30-40 minutes total. This long, slow caramelization is the foundation of the dish's rich flavor—I never rush this step because those browned bits create the soul of the sauce. Once the onions reach a deep golden color, preheat your oven to 325°F and add the minced garlic to the pot, stirring constantly for about 1 minute until fragrant.

Sprinkle the 3 tablespoons of all-purpose flour over the onion and garlic mixture, stirring well to coat everything evenly. Cook for 1 minute, stirring constantly, to lightly toast the flour and eliminate any raw taste. Pour in the dry red wine, scraping the bottom of the pot with a wooden spoon to deglaze and release all the browned bits stuck to the pan—these bits are pure concentrated flavor. Add the beef broth, 5 sprigs of fresh thyme, 2 bay leaves, and 1 tablespoon of Worcestershire sauce, stirring to combine everything into a rich, aromatic sauce.

Return the seared beef roast to the pot, nestling it into the onion and sauce mixture so it's mostly submerged. Bring everything to a gentle simmer over medium heat, then cover the Dutch oven with a lid and transfer it to your preheated 325°F oven. Braise for 3 to 3.5 hours, until the meat is fork-tender and easily pulls apart—I like to check it at the 2.5-hour mark just to gauge how it's progressing, but avoid opening the lid too frequently or you'll lose heat and moisture.

Remove the pot from the oven and carefully take out the thyme sprigs and bay leaves—tweezers or a slotted spoon make this easier. Using two forks, shred the tender roast directly in the pot, breaking it into bite-sized pieces and mixing it gently back into the rich onion sauce. Taste and adjust seasoning with additional salt and pepper if needed. Serve the pot roast hot in shallow bowls with plenty of the caramelized onions and sauce spooned over top.