01 - Heat 1 tablespoon oil in a large pot or Dutch oven over medium heat. Add diced onion and celery; sauté for 4 minutes until softened. Add minced garlic and cook for 1 minute more until fragrant.
02 - Add sliced carrots and diced potatoes to the pot. Stir to coat with the aromatic mixture, cooking for another 2 minutes to lightly season the vegetables.
03 - Pour in 6 cups beef broth and 2 cups water. Add bay leaf, dried thyme, and black pepper. Bring to a boil, then reduce heat to establish a gentle simmer.
04 - Cover pot and simmer for 20 minutes, until potatoes are just tender when pierced with a fork.
05 - Stir in chopped cabbage and continue cooking uncovered for 15 to 20 minutes, until cabbage is wilted and tender.
06 - Add diced or shredded corned beef to the pot. Simmer for 10 minutes to heat through and allow flavors to meld together.
07 - Taste broth and adjust seasoning with salt as needed. Remove and discard bay leaf. If desired, stir in Dijon mustard for added tanginess.
08 - Ladle hot soup into bowls, garnish with fresh parsley, and serve immediately with crusty bread.