Pat the chuck roast very dry with paper towels, trim any large hard chunks of fat, and season all sides evenly with the kosher salt and black pepper.
Heat the olive oil in a large cast iron skillet over medium high heat and sear the roast for 3 to 4 minutes per side, including the edges, until a deep golden brown crust forms. Transfer to a plate.
Pour the beef broth into the skillet and scrape up the browned bits, then whisk the hot broth with the beef base, onion soup mix, Worcestershire sauce, tomato paste, and dried thyme until smooth.
Spray the slow cooker or line it, then layer the onion wedges, garlic, carrots, celery, and halved potatoes in an even, snug layer.
Set the seared roast on top of the vegetables, tuck the bay leaves alongside, and slowly pour the broth mixture over the roast and vegetables. Cover with the lid.
Cook on LOW for 8 to 10 hours or on HIGH for 5 to 6 hours, until the roast shreds easily with a fork and reads around 200 degrees Fahrenheit. Avoid lifting the lid.
Lift the roast onto a cutting board, cover loosely with foil, and rest for 10 minutes. Arrange the vegetables on a platter and discard the bay leaves from the cooking liquid.
Strain the cooking liquid through a fat separator, simmer it in a saucepan, and whisk in the cornstarch slurry. Cook for 2 to 3 minutes until thickened, then stir in the browning sauce and season to taste.
Pull the roast into big juicy chunks or slice against the grain, spoon hot gravy over the beef and vegetables, sprinkle with fresh parsley, and serve right away.