Combine onion, celery, peppers and butter in 3-quart saucepan; cook until over very low heat until softened. Add flour; cook a few minutes, stirring occasionally. Add bay leaf and stocks; bring to boil. Reduce to simmer.
Cut corned beef into julienne strips; add to soup. Add cheese and stir slowly until melted. Add sauerkraut, then roux. Let simmer 30 minutes. Add half-and-half and heat through. Remove bay leaf. Ladle soup into bowls and top with croutons.