In a large saucepan, cook onion and celery in butter until tender; stir in flour until smooth. Gradually stir in water and bouillon, and bring to a boil. Reduce heat to low, and simmer for 5 minutes.
Stir in corned beef, sauerkraut, cream, and 1 cup of the cheese. Cook and stir for 30 minutes or until slightly thickened.
Ladle soup into 8 ovenproof bowls. Top each serving with a slice of bread and sprinkle 1/4 cup of the cheese on top of each slice of bread. Place in oven under broiler until the cheese melts and lightly browns.