Cut the beef into 6 large chunks (one per serving) and place them in a large saucepan. Pour in the beef broth and water. Bring to a boil over high heat, then simmer covered on medium heat until nearly tender, for 1 to 1 1/2 hours.
Combine polenta, potatoes, and onion into the stew, stirring gently. Cover and simmer for 15 minutes. Add pumpkin, corn, carrot, bell pepper, celery, and leek to the stew. Simmer until vegetables are tender, adding water if needed. Stir in oregano and paprika in the last 5 minutes.
Season the dish with a pinch of salt and freshly ground pepper. Serve in bowls and garnish with freshly chopped cilantro.