Prepare the beef: Trim any large, hard pieces of fat from the chuck roast, then cut the beef into 1½-inch cubes.
Season the beef: Pat the beef completely dry with paper towels. Toss with the sea salt, black pepper, garlic powder, onion powder, and smoked paprika until evenly coated.
Sear the beef: Heat 2 tablespoons of the oil in a large skillet over medium-high heat until very hot. Sear the beef in 3 batches, leaving space between the pieces. Cook for 2–3 minutes per batch without stirring, until deeply browned on at least 2 sides. Transfer each batch to the slow cooker.
Cook the onion: Reduce the heat to medium. Add the remaining tablespoon of oil and the onion. Cook for 4–5 minutes, scraping up the browned bits as the onion releases moisture.
Add the aromatics: Add the garlic and tomato paste. Cook for 1 minute, stirring, until the tomato paste darkens slightly.
Deglaze the skillet: Pour in the beef broth and scrape the bottom of the skillet with a wooden spoon to release the remaining browned bits. Bring the mixture to a simmer, then pour it over the beef in the slow cooker.
Add the rest of the seasonings: Stir in the Worcestershire sauce and Dijon mustard. Add the thyme and bay leaves.
Slow cook: Cover and cook on LOW for 6–7 hours or HIGH for 4–5 hours.
Check the beef: The beef is ready when a piece falls apart under light pressure from a fork. If it still feels firm, springy, or chewy, continue cooking and check again after about 45 minutes.
Make the slurry: Remove the bay leaves. Whisk the cornstarch and cold water together in a small bowl until completely smooth.
Thicken the gravy: Stir the slurry into the slow cooker. Cover and cook on HIGH for 20–30 minutes, or until the gravy is thick enough to coat the back of a spoon.
Serve: Taste and adjust the salt and black pepper as needed. Serve over mashed potatoes, egg noodles, or rice and top with chopped parsley.
Notes
If the beef is tough: Chuck roast usually needs more time when it is chewy or firm. Continue cooking until the pieces are fork-tender.
Keep the pieces large: Cut the chuck into about 1½-inch cubes so the beef stays in distinct, tender pieces instead of shredding during the long cook.
Use cold water for the slurry: Whisk the cornstarch into cold water until smooth before adding it to the hot gravy to prevent lumps.
LOW or HIGH works: Both settings were tested. The beef can be tender by the 4-hour mark on HIGH, but some pieces may need closer to 5 hours. Cook until the beef is fork-tender rather than relying only on the clock.