Prepare the Slow Cooker: Lightly coat the inside of a 6-quart slow cooker with nonstick cooking spray. Place the halved potatoes and onion pieces in the bottom. This initial step ensures that the vegetables do not stick to the bottom and helps them cook evenly.

Season the Vegetables: Sprinkle the potatoes and onions with 1/2 teaspoon salt and 1/4 teaspoon black pepper. Toss gently so the seasoning is distributed throughout the vegetables. Properly seasoning the vegetables enhances their flavor and allows them to absorb the savory broth during cooking.

Season the Beef: Place the beef chuck pieces in a large bowl. Add the remaining 1 teaspoon salt, remaining 1/2 teaspoon black pepper, smoked paprika, thyme, and rosemary. Toss until each piece is evenly coated. This step is crucial for ensuring that every bite of beef is flavorful and aromatic.

Arrange the Beef: Spread the seasoned beef over the potatoes and onions. Keep the meat in a relatively even layer rather than packing it tightly into one mound. This arrangement allows for even cooking and helps the beef to become tender without steaming.

Mix the Cooking Liquid: In a medium bowl, whisk together the beef broth, Worcestershire sauce, tomato paste, Dijon mustard, and minced garlic. Continue whisking until the tomato paste is mostly dissolved. This mixture will provide a rich flavor base for the dish.

Add the Liquid: Pour the broth mixture evenly over the beef. The ingredients will not be fully submerged; they will release more liquid as they cook. This is normal, as the slow cooking process will create a flavorful broth.

Slow-Cook Until Tender: Cover and cook on low for 6 1/2 to 7 hours. Avoid opening the lid during the first 6 hours because heat and steam escape each time it is lifted. Keeping the lid closed ensures that the beef and potatoes cook evenly and become tender.

Check the Beef and Potatoes: Test a large piece of beef and one of the thickest potato pieces. The potatoes should be easily pierced with a knife, and the beef should separate with gentle pressure from a fork. Beef is safe at 145°F (63°C), but chuck usually becomes properly tender at a substantially higher temperature after prolonged cooking.

Prepare the Cornstarch Slurry: Whisk the cornstarch and cool water together in a small bowl until completely smooth. Do not add dry cornstarch directly to the slow cooker because it will form lumps. This slurry is essential for thickening the gravy without clumps.

Thicken the Gravy: Stir the slurry into the hot cooking liquid, gently moving the beef and potatoes so the mixture is distributed. Cover and cook on high for 15 minutes, or until the gravy lightly coats the back of a spoon. This step transforms the cooking liquid into a rich gravy.

Adjust the Consistency: If the gravy is still thin, cook uncovered on high for another 5 to 10 minutes. If it becomes too thick, stir in 2 tablespoons (30 ml) of warm beef broth or water. Adjusting the consistency ensures that the gravy is just right for serving.

Rest and Serve: Turn off the slow cooker and let the steak and potatoes stand uncovered for 5 minutes. Taste the gravy and adjust the seasoning only if needed. Garnish with chopped parsley, if using, and serve warm. Allowing the dish to rest helps the flavors meld and makes for a better presentation.