Cut the piece of beef into 6 large chunks (one per serving). Place the beef into a large saucepan; pour in the beef stock and water. Bring to the boil over high heat, then reduce heat to medium, cover and simmer until nearly tender, 1 to 1 1/2 hours.
Stir the polenta into the stew along with the potatoes and onion. Cover and simmer for 15 minutes. Cut the pumpkin into 6 serving-sized pieces, and add to the stew along with the sweetcorn, carrot, red pepper, celery and leek; simmer until the vegetables are tender, adding more water if needed to barely cover. Stir in the oregano and paprika during the last 5 minutes.
Season to taste with salt and pepper. Ladle into serving bowls, and sprinkle with chopped coriander.