Pat the chuck roast completely dry using paper towels. Season generously on all sides with salt and pepper.

Heat the olive oil and 1 tablespoon of butter in a large skillet or Dutch oven over medium-high heat. Sear the roast for 3-4 minutes per side until a deep brown crust forms. Remove the beef and set aside.

Reduce the heat to medium-low. Add the remaining butter to the pan. Add the thinly sliced yellow onions. Cook slowly, stirring occasionally, for 20-30 minutes until deeply golden brown and sweet.

Add the minced garlic and thyme to the onions and cook for 1 minute until fragrant. If using red wine, pour it in and scrape up any browned bits from the pan. Let the wine reduce by half, about 5 minutes. Add the beef broth, bay leaves, and Worcestershire sauce. Bring to a gentle simmer.

Transfer the caramelized onions and broth mixture to the slow cooker. Place the seared chuck roast directly on top of the onions.

Cook on LOW for 8-10 hours, or on HIGH for 4-5 hours, until the beef is fork-tender.

Remove the roast to a cutting board and shred using two forks. Discard the bay leaves. For thicker gravy, mix cornstarch or flour with a little cold water to make a slurry, stir into the hot broth, and simmer for 5-10 minutes. Return the shredded beef to the gravy.

Ladle the shredded beef and gravy into oven-safe bowls or a broiler-safe pan. Top generously with shredded Gruyere cheese. Broil for 2-3 minutes until the cheese is bubbly, golden, and melted.

Serve immediately with suggested sides like mashed potatoes or crusty bread.