Load the vegetables first: put the potato and carrot chunks in the bottom of a 5–6 quart slow cooker. Vegetables take longest to soften, and sitting at the bottom in the cooking liquid helps them cook evenly.
Add the beef: arrange the chuck chunks on top of the vegetables. Season with the black pepper.
Mix the gravy base: whisk the condensed soup, onion soup mix, beef broth and tomato paste (if using) in a bowl until smooth, then pour it evenly over the beef and vegetables. Do not add extra salt yet — the soup mix is salty, and you can adjust at the end.
Cook low and slow: cover and cook on LOW for 7–8 hours or on HIGH for 4–5 hours, until the beef is fork-tender and the potatoes are soft all the way through. Try not to lift the lid during cooking.
Thicken if you like: for a thicker gravy, stir in the cornstarch slurry during the last 30 minutes on HIGH.
Rest and serve: turn the cooker off, leave the lid off for 10 minutes, taste and add salt only if needed, scatter with parsley and serve warm.