Prepare the slow cooker: Place the corned beef, potatoes, celery, onion, carrots, and cabbage into the slow cooker.
Mix the broth: In a medium bowl, whisk together the beef broth, Worcestershire sauce, pickling spice, garlic powder, paprika, salt, and pepper. Pour this mixture evenly over the meat and vegetables.
Cook: Cover and cook on low for 8–10 hours or on high for 5–6 hours, until the meat is tender and the vegetables are fully cooked.
Optional thickening: If you prefer a thicker broth, mix the cornstarch with a little cold water to make a slurry, then stir it into the slow cooker during the last 30 minutes of cooking.
Finish and serve: Adjust seasoning with extra salt and pepper to taste. Garnish with freshly chopped parsley and serve warm.
