Melt butter in a large saucepan over medium-high heat. Cook and stir onion and celery in hot butter until tender. Mix in flour until smooth. Gradually stir in water and bouillon; bring to a boil. Reduce heat to low and simmer for 5 minutes.

Stir in cream, 1 cup cheese, corned beef, and sauerkraut. Cook and stir until soup is slightly thickened, about 30 minutes.

Meanwhile, preheat the oven's broiler.

Ladle soup into 8 ovenproof bowls. Top each serving with a bread slice; sprinkle each with 1/4 cup cheese.

Place under the preheated broiler until cheese is melted and lightly browned.