Preheat the Oven:Preheat your oven to 325°F (163°C).
Sear the Beef: Pat the chuck roast (3 pounds) completely dry with paper towels and season generously with salt and pepper. In a large Dutch oven, heat the vegetable oil (2 tablespoons) over medium-high heat until shimmering. Sear the roast 4–5 minutes per side until deeply browned. Remove to a plate and set aside.
Caramelize the Onions: Lower the heat to medium. Add the butter (4 tablespoons) to the pot. When melted, add the sliced onions (4) and a pinch of salt. Cook, stirring occasionally, for 25–30 minutes, scraping up fond, as needed, until onions are rich golden brown and jammy.
Add Garlic & Tomato Paste: Stir in the minced garlic (1 tablespoon) and cook for 1 minute until fragrant. Add tomato paste (2 tablespoons) and cook 2–3 minutes, stirring, until it darkens slightly and smells sweet and rich.
Deglaze with Wine: Pour in the wine (1 cup) and stir, scraping up all the browned bits from the bottom of the pot. Let simmer 2–3 minutes until slightly reduced.
Build the Braising Liquid: Stir in the beef broth, Better Than Bouillon (1 heaping tablespoon), Worcestershire sauce (2 tablespoons), soy sauce (1 tablespoon), and balsamic vinegar (1 tablespoon). Add thyme (4-6 sprigs), rosemary (1 sprig), and bay leaf (1). Return the beef to the pot, nestling it into the onions and liquid (it should be about halfway submerged).
Braise: Cover and transfer to the preheated oven. Cook for 3 hours, or until the beef is fall-apart tender. Halfway through, flip the roast and spoon the onions over top. In the final 20 minutes, uncover the pot so the sauce can thicken slightly.
Shred and Broil: Remove the bay leaf, thyme, and rosemary. Using two forks, gently shred the beef into large chunks. Arrange the toasted baguette slices on top and sprinkle evenly with Gruyère cheese.
Broil to Finish: Broil uncovered for 2–4 minutes, watching closely, until the cheese is melted, bubbly, and golden brown.
Serve: Spoon generous portions of beef, onions, and cheesy bread into bowls. Serve immediately while bubbling hot.