Move the oven rack to the top position and turn the oven broiler to high. (*Note 3)
Pat dry the chuck roast with paper towels and coat with the beef bouillon paste. Evenly sprinkle the roast with kosher salt, black pepper and transfer to a roasting pan.
Place the pan under the broiler and let roast sear for 5-8 minutes, until the outside is dark brown and crusty. Keep your eyes on it, as it can quickly burn.
Remove the roast from the oven and reduce the oven temperature to 300°F. Add the sliced onions and thyme sprigs around the roast. In a small bowl, add the beef broth and Worcestershire sauce and stir to combine. Pour the mix over the onions then spread the minced garlic on the top of the roast.
Tightly cover the roasting pan with aluminum foil and transfer the pan to the oven. Bake for 3½-4 hours, or until fork tender. Check at the 3½ hour mark, to see if it shreds easily with two forks. If not, bake for another 30 minutes, or until it does easily shred.
Remove the roasting pan from the oven and let rest, still covered, for 10-15 minutes. When ready to serve, remove the foil and use two forks to shred the roast right into the juices.
Notes
Note 1 - If you don't have beef bouillon, use olive oil instead.Note 2 - You can use oregano or rosemary instead, or omit all together.Note 3 - If using a dutch oven you may need to place the oven rack in the second position as they're much taller than a roasting pan.