Slice onions and season the chuck roast generously with salt and pepper.

Heat olive oil in a large skillet over medium-high heat. Sear the roast on all sides until browned (about 5 minutes per side).

In the same skillet, add sliced onions and cook over medium heat until caramelized (about 25 minutes), stirring occasionally.

Layer the seared roast in a slow cooker or Dutch oven. Top with caramelized onions, beef broth, Worcestershire sauce, and thyme.

Cover and cook on low for 6 hours or until fork-tender.

In the last few minutes of cooking, sprinkle Gruyère cheese on top and let it melt before serving.